| A a an international symbol for year, taken from the Latin word annus. Although English-speaking countries will certainly continue to use the traditional symbol yr for most purposes, scientists use the a symbol in papers and textbooks. The symbol is often seen in combinations such as Ma (million years) or Ga (billion years). A* a unit used to measure the wavelength of X-rays. Because it's easier to measure the ratio between two X-ray wavelengths than it is to measure the wavelengths themselves, the wavelengths are usually stated as multiples of a standard wavelength. The X unit and A* are the units used for this purpose. A* was defined by J.A. Bearden in 1965 to provide a unit approximately equal to the angstrom (10-10 meter or 0.1 nanometer). Later measurements have shown that in fact A* is equal to approximately 1.000 002 x 10-10 meter. ab- a prefix indicating that an electrical unit is part of the CGS absolute electromagnetic system. These units are also indicated by the notation emu (as in "volt emu"). Although these units are defined naturally as part of the CGS system, all of them except the abampere and abcoulomb are either much too large or much too small for most applications. They have been replaced almost completely by the corresponding SI units. Following are the SI equivalents for each of the "ab" units: Electric current: 1 abampere = 10 amperes Electric charge: 1 abcoulomb = 10 coulombs Capacitance: 1 abfarad = 109 farads = 1 gigafarad Inductance: 1 abhenry = 10-9 henry = 1 nanohenry Resistance: 1 abohm = 10-9 ohm = 1 nanohm Conductance: 1 abmho = 109 siemens Magnetic flux density: 1 abtesla = 10-4 tesla = 1 gauss Potential: 1 abvolt = 10-8 volt = 10 nanovolts Power: 1 abwatt = 10-7 watt = 0.1 microwatt Magnetic flux: 1 abweber = 10-8 weber = 1 maxwell. acre (ac or A) a unit of area used for measuring real estate in English-speaking countries. "Acre" is an Old English word meaning a field. Originally intended as the area that could be plowed in a day by a yoke of oxen, the acre was standardized, at least by the year 1200, to be the area of a field one furlong (40 rods or 10 chains) long by 4 rods (or 1 chain) wide. Thus an acre is 10 square chains, 160 square rods, 43 560 square feet or 4840 square yards. There are exactly 640 acres in a square mile. In metric countries the unit corresponding to the acre is the hectare, which is 10,000 square meters (the area of a square 100 meters on each side). One acre is equal to 0.404 687 3 hectare. acre foot (ac ft) a unit of volume used to measure the capacity of reservoirs. One acre foot is a volume one foot deep covering an area of one acre. Thus an acre foot contains 43 560 cubic feet, or about 1233.489 cubic meters. acre inch (ac in) a unit of volume, equal (of course) to 1/12 acre foot. An acre inch contains 3630 cubic feet, or about 337.24 cubic meters. AD abbreviation for the Latin anno domini, "year of the Lord," the traditional designation for years of the common or Christian era. This abbreviation is often replaced by CE (common era), especially in countries where Christianity is not a dominant religion. -ad a suffix added to a number to create a unit of quantity equal to that number: for example, a 24ad is a unit of quantity equal to 24. Units of quantity equal to 1 through 8 are known, respectively, as the monad, dyad, triad, tetrad, pentad, hexad, heptad, and octad, terms coined by adding -ad to the Greek numbers 1-8. Admiralty mile see nautical mile. aeon a unit of time equal to one billion years (1 Ga). Proposed for use in geology, the aeon is not approved by the SI and hasn't found much favor. agate a traditional unit of distance used in printing. The agate is usually considered equal to 1/14 inch (1.814 millimeters), since the traditional type size called agate set 14 lines to the inch (very small print!). In the more modern measuring system based on points [2], agate type has a height of 5.5 points; this would make the agate equal to about 0.076 inch or 1.933 millimeters. air watt an engineering unit used to express the effective cleaning power of a vacuum cleaner or central vacuum system. The air watt is practically the same as the ordinary watt. Measurements of vacuum power, however, are computed from English units using the following formula established by the American Society for Testing and Materials (ASTM): power in air watts equals 8.5¡¤F¡¤S, where F is the air flow in the system in cubic feet per minute (CFM) and S is the suction pressure in inches of water column (in WC). This definition makes the air watt equal to 0.9992 watt. almude a traditional unit of volume in Spain and Portugal. The two countries used the name for units of quite different sizes. The Spanish almude is comparable to the British gallon; it holds about 4.625 liters, equivalent to 1.017 imperial gallon or 1.222 U.S. liquid gallon. The Portuguese almude is much larger; it holds about 16.7 liters, which is 3.67 imperial gallons or 4.41 U.S. liquid gallons. aln or alen a traditional Scandinavian unit of distance very similar to the north German elle: roughly 2 feet or 60 centimeters. alpha TE an abbreviation for "alpha tocopherol equivalent," a measure of vitamin E used in nutrition. Vitamin E is actually a group of related chemical compounds called tocopherols. The activity, or potency, of vitamin E in a food or food supplement is measured by the quantity (in milligrams) of alpha tocopherol (the most active of the forms of the vitamin) which would be equivalent to the compounds present in the food or supplement. One milligram alpha TE is equal to 1.5 international units (IU). AM or am abbreviation for the Latin ante meridiem, "before noon," used after a time to indicate that the time is before 12:00 (noon). amagat units used by physicists to express the relative volume and density of gases. The amagat volume unit is about 22.414 liters per mole (L/mol) or 0.022 414 L/m3, the volume occupied by a gas at standard temperature (0.01¡ÆC) and standard pressure (1 atmosphere). The amagat density unit represents the corresponding relative density, which is equal to one kilomole per standard volume, or 44.615 mole per cubic meter (mol/m3), or 0.044 615 mole per liter (mol/L), again provided the measurement is made at standard temperature and pressure. In general, the ideal gas law shows that the relative density is equal to P/RT, where P is the pressure on the gas, T is the absolute temperature (in kelvins) and R is the universal gas constant, R = 8.314 joules per mole per kelvin. The unit honors the Dutch physicist E. H. Amagat (1841-1915), whose work included the study of gases under pressure. amber an old English unit of volume, used for both liquids and dry goods. The amber was equal to about 4 bushels or roughly 140 liters. American run see run. ampere (A or amp) the fundamental SI unit of electric current, named for the French physicist Andre-Marie Ampere (1775-1836), one of the pioneers in studying electricity. The official definition of the ampere goes like this: suppose we have two parallel conductors, infinitely long and having negligible cross section. Place these conductors one meter apart in a perfect vacuum. One ampere is the current which, if it's flowing in these conductors, creates between them a force of 0.2 micronewtons per meter of length. (You're welcome to object that no one can make an infinitely long conductor, nor a perfect vacuum. But scientists can use the idealized definition to construct appropriate real-world equipment in their laboratories.) The other electrical units are all defined in terms of the ampere. For example, one ampere represents a current flow of one coulomb of charge per second. One ampere of current results from a potential distribution of one volt per ohm of resistance, or from a power production rate of one watt per volt of potential. ampere hour (A h or amp hr ) a commercial unit of electric charge, often used to state the capacity of a battery. One ampere hour is the charge accumulated by a steady flow of one ampere for one hour. This is equivalent to 3600 coulombs. ampere turn (At) the MKS unit of "magnetomotive force." Electric current passing through a coil of wire generates a magnetic field. This field-generating ability is called magnetomotive force; it is equal to the product of the current, in amperes, and the number of turns of wire in the coil. One ampere turn equals 4pi/10 = 1.256 637 gilberts (Gb). amphora a historic unit of volume. An amphora is the volume of an urn or jar of the same name. These urns were tall, with handles near the top on both sides (the word amphora comes from two Greek words meaning "on both sides" and "carry"). Amphoras were the containers of choice for shipping wine and many other commodities in the ancient world. Archaeologists report that the Greek amphora held about 38.8 liters (10.25 U.S. liquid gallons, or 8.54 British imperial gallons). The Roman amphora was smaller, about 25.5 liters (6.74 U.S. gallons or 5.61 British imperial gallons). angstrom (A or A) a metric unit of length, equal to 0.1 nanometer or 10-10 meter. Angstroms are used most often to measure the wave length of light waves. There is a technique called spectroscopy for identifying chemical substances by the wave lengths of light which they absorb (or emit, depending on the circumstances). Light shining through or from the substance is passed through a prism, which separates the various wave lengths to form a colorful spectrum. The spectrum often includes bright bands corresponding to wave lengths at which the substance emits light, or dark bands if the substance is absorbing light. The pattern of bands identifies the substance in somewhat the same way a fingerprint identifies a person. One of the pioneers of spectroscopy was the Swedish physicist Anders Jon Angstrom (1814-1874), and in his honor the wave lengths of light waves are customarily stated in angstroms. Although English speakers usually pronounce the word as if it were English, the Swedish pronunciation is closer to "ong-strerm." angular mil see mil [2]. animal unit (AU) a unit of feed consumption used in U.S. dairying and ranching. One animal unit is the feed or grazing requirement of a mature cow weighing 1000 pounds (453.59 kilograms). Total feed requirements are often figured by the animal unit month (AUM), the feed required to sustain one animal unit of livestock for one month. anker a traditional unit of volume for wine and other alcoholic beverages, used in Britain and elsewhere in northern Europe. The anker holds about 38.6 liters (almost the same as a Greek amphora). This about 10.2 U.S. liquid gallons or British wine gallons, and about 8.5 British imperial gallons. The word anker is of Dutch origin. annual percentage rate (% APR) a unit used in the U.S. for stating interest rates and rates of return on investment. By federal regulation, these rates can be stated however a financial institution wishes, but they must be stated also in % APR so that consumers can compare rates of different loans and investment opportunities. Mathematically, the natural rate r of return on money is the "instantaneous" rate, the rate that allows for compounding of interest continuously. The APR is the percentage growth rate a of the money over a period of one year, as if interest were compounded annually. The two rates are related by the formulas a = er - 1 and r = ln (1 + a), where er is the natural exponential function and ln is the natural logarithmic function. Apgar score a numerical measure of the health of a newborn baby. One minute after birth (and at regular intervals thereafter through the first moments of life) newborns are rated 0, 1, or 2 on five indicators of health (respiratory effort, heart rate, skin color, muscle tone, and reflexive response to smell). Possible scores therefore range from to 10. The unit is named for its inventor, the American anesthetist Virginia Apgar. apostilb (asb) an MKS unit of luminance, representing the brightness of a surface uniformly radiating 1 lumen per square meter. This is the brightness produced by 1/pi candela or 1 lux of light. Since the apostilb basically measures the same situation as the lux, it is rarely used. arcminute (' or min) a unit of angular measure equal to 60 arcseconds and to 1/60 degree. There are 21 600 arcminutes in a circle. arcsecond (" or as or sec or s) a unit of angular measure equal to 1/60 arcminute. One arcsecond is a very small angle indeed: there are 1 296 000 seconds in a circle. The SI defines s as the symbol for the time unit and recommends " as the symbol for the arcsecond. The symbol "as" has become common in astronomy, where very small angles are stated in milliarcseconds (mas). are (a) a basic unit of area in the metric system, equal to 100 square meters. The word is pronounced the same as "air." Being the area of a square 10 meters on each side, the are is a little large for measuring areas indoors and a little small for measuring areas outdoors. As a result, the are is not used as often as its multiple, the hectare (ha). One are is approximately 1076.39 square feet or 0.02471 acre. arpent [1] a traditional unit of distance in French-speaking countries. The arpent equals 30 toises or 10 perches; this is about 191.8 feet or 58.47 meters. The unit was used to measure land; in fact, arpentage is the French word for surveying. In Canada the arpent has an official definition: 191.835 English feet (58.471 308 meters). arpent [2] a traditional unit of area in French North America (Quebec and Louisiana), equal to one square arpent [1]. The arpent of area equals 900 square toises, 100 (square) perches, approximately 0.8445 acre or 0.3419 hectare. By the official Canadian definition, the arpent of area contains 36 800.667 23 English square feet or about 0.844 827 acre (0.341 889 hectare). arratel or artel versions of a traditional Arab unit of weight; see rotl. The spelling arratel is used, for example, in Portugal, where the arratel is an alternate name for the libra. arroba a traditional unit of weight in Spain and Portugal, equal to 1/4 quintal. However, the Spanish and Portuguese quintals are of different sizes. In Spain, the arroba equals 25.36 pounds (11.50 kilograms); arrobas of very similar sizes were established in the Spanish speaking countries of Latin America. In Portugal and Brazil, the arroba equals 32.38 pounds (14.69 kilograms). The arroba has also been used as a metric unit equal to exactly 15 kilograms. The name of the unit comes from ar rub', Arabic for "the quarter." arshin a traditional Russian unit of distance. Peter the Great standardized the arshin at exactly 28 English inches, or 0.7112 meter, early in the 1700s. The arshin was also used in several other countries adjacent to Russia. The arshin is also used as a unit of area equal to one square arshin; this would be equal to 5.4445 square feet or 0.5058 square meter. ASA number for many years, the initials of the American Standards Association appeared on film packages in the United States as a measure of the speed of the photographic emulsion (the stuff on the film that "develops" to form the picture. The scale is arbitrary, but the important thing to know is that the speed at which the image registers on the film is proportional to the ASA number. Thus ASA 400 film registers an image twice as fast as ASA 200 film and four times as fast as ASA 100 film. The ASA number is now combined with the European DIN rating as a composite ISO rating. For example, ASA 400 film is now marked ISO 400/27¡Æ, because 27 is the DIN rating corresponding to ASA 400. assay ton (AT) a specialized unit of mass used by minerologists in assaying (testing) ores for the presence of gold, silver, platinum, uranium, or other valuable metals. One assay ton equals 29.167 grams: just a little over an an ounce! The assay ton is actually a sample size. Since there are 29 167 troy ounces in a short ton (2000 lb), the number of milligrams of a precious metal in a sample of one assay ton is numerically equal to the number of troy ounces of that metal in one ton of raw ore. In Britain, the assay ton is based on the long ton and thus equals 32.667 grams. astronomical unit (ua or au) a unit of distance used by astronomers to measure distances in the Solar System. One astronomical unit equals the average distance from the center of the Earth to the center of the Sun. The currently accepted value, adopted in 1996, is 149 597 870 691 meters (1.495 978 706 91 x 108 kilometers or about 92 955 807 miles), with an uncertainty of about 30 meters. The astronomical unit is a convenient yardstick for measuring the distances between objects in the Solar System. Astronomers find it particularly convenient to use astronomical units in solving the equations of planetary motion. Because these equations are the same regardless of the unit used for distance, the predictions they generate will remain correct even if future astronomers determine a slightly different length for the distance between the Earth and Sun. The official symbol for the unit is ua, but the symbol au is also used. This unit is accepted for use with SI units. atmo-meter (atmo-m) a unit used in atmospheric physics to compare the "depth" or total volume of atmospheres, or components of atmospheres. The depth (in atmo-meters) is equal to the depth (in meters) the atmosphere, or one gas component of the atmosphere, would have if it formed a uniform layer at standard temperature (0.01¡ÆC) and pressure (1 atm). One atmo-meter represents 2.686 99 x 1025 molecules of gas per square meter of planetary surface. at symbol for the technical atmosphere. atmosphere (atm or atmos) a unit of pressure designed to equal the average pressure of the Earth's atmosphere at sea level. In other pressure units, one atmosphere equals exactly 1013.25 millibars (mb), 101.325 kilopascals (kPa), approximately 29.92 inches of mercury (in Hg), or 14.7 pounds of force per square inch (lb/in2). This is the standard atmosphere; it equals 1.0332 technical atmosphere. atomic mass unit (u or amu) the unit of mass used by chemists and physicists for measuring the masses of atoms and molecules. Early in the nineteenth century, scientists discovered that each chemical element is composed of atoms, and that each chemical compound is composed of molecules in which atoms are combined in a fixed way. No one knew then just how small atoms and molecules really are, but as long as the relative weights of the different atoms were known, the outcome of chemical reactions could be predicted. These relative masses were determined by careful study of various reactions. The general idea was that atoms of hydrogen, known to be the lightest element, should have a mass of 1 amu, and all the other atoms should have masses which are whole-number multiples of this (then unknown) mass of the hydrogen atom. For a long time, physicists and chemists disagreed on the details of this definition. In 1960 they agreed on the definition of the unified atomic mass unit as 1/12 the mass of the most common atoms of carbon, known as carbon-12 atoms. (Most elements are mixtures of atoms which have different masses because they contain different numbers of neutrons; these varieties are called isotopes.) Careful experiments have measured the size of this unit; the currently accepted value (1998) is 1.660 538 73 x 10-24 grams. (This number equals 1 divided by Avogadro's number; see mole.) In addition, 1 amu equals approximately 931.494 MeV (see electron volt). atomic number The atomic number was originally defined (about 1865) simply as an index describing the position of an element in the periodic table. Not until 1913 was it known that the atomic number is actually a unit of measurement, equal to the number of electrons surrounding a neutral (uncharged) atom, and also the number of protons in the nucleus. atto- (a-) a metric prefix denoting 10-18 (one quintillionth). For example, one electron volt equals 0.1602 attojoule. The root of the prefix is atten, the word for 18 in Danish and Norwegian. AUC abbreviation for the Latin phrase ab urbe condita, "from the founding of the city." The Romans counted years from the legendary founding of Rome by Romulus and Remus, an event placed in 753 BC (or BCE) in our present calendar. The Christian Church continued to count years AUC for centuries after the fall of Rome. Our year 1999 is 2752 AUC. aume an old English wine measure equal to about 40 gallons (roughly 150 liters). The aume is the English version of a German unit, the ohm [2]. aune a traditional French unit of distance, varying from region to region but similar in length to the English ell. B informal abbreviation for "billion," generally meaning the American billion 109. This abbreviation is non-metric: the metric abbreviation for 109 is G, standing for the prefix giga-. The B form has been used in such units as Bcf (billion cubic feet) and BeV (billion electron volts). bag [1] another name for a sack, used as unit of measure in Britain. bag [2] a unit of weight for cement. Traditionally a bag of portland cement weighs 94 pounds (42.6 kilograms) in the US. and 87.5 pounds (39.7 kilograms) in Canada. However, cement is now being sold also in metric-sized bags of 50 kilograms (110.2 pounds) by many suppliers. baker's dozen an informal unit of quantity, equal to 13. Bakers often toss in an extra item for each dozen bought, making a total of 13. The custom apparently arose during a time when English bakers were subject to a fine if they failed to deliver a full dozen, so they included 13 just to be safe. bale (bl) [1] a bundle of merchandise, usually pressed and bound in some way. The word "bale" has been used in many ways to describe standard packages of various commodities. For example, a bale of paper is traditionally equal to 10 reams. In agriculture, a bale of hay is generally a huge round bundle left in the field until needed; these bales can weigh up to 1500 pounds (700 kilograms). In U.S. garden shops, a bale of straw is typically 3 cubic feet (0.085 cubic meter). bale (bl) [2] a commercial unit of weight to measure shipments of cotton. In the United States, one bale of cotton, formerly equal to 500 pounds, is now equal to 480 pounds. The British used the Egyptian bale, formerly equal to 750 pounds but now equal to 720 pounds. Other countries use a variety of cotton bale weights. Balling a unit of density; see degree Plato. balthazar a large wine bottle holding about 12 liters, 16 times the volume of a regular bottle. bank cubic meter (BCM) a traditional unit of volume in coal mining. A bank cubic meter represents the contents of a cubic meter of rock in place, before it is drilled and blasted. bar (b) a metric unit of atmospheric pressure, equal to one million dynes per square centimeter, 100 kilopascals, 750.062 torr, or about 14.503 78 pounds per square inch. The word comes from the Greek baros, "weighty." We see the same root in our word, barometer, for an instrument measuring atmospheric pressure. One bar is just a bit less than the average pressure of the Earth's atmosphere, which is 1.013 25 bar. In practice, meteorologists generally record atmospheric pressure in millibars (mb). In English-speaking countries, barometric pressure is also expressed as the height, in inches, of a column of mercury supported by the pressure of the atmosphere. In this unit, one bar equals 29.53 inches of mercury (in Hg). barge an informal unit of volume used in the U.S. energy industry. The barges used on American rivers customarily carry about 25 000 barrels of oil (see barrel [2] below). This is equivalent to 1.05 million gallons, roughly 1400 register tons, or about 3975 cubic meters. barleycorn an old English unit of length, equal to 1/3 inch, or about 8.5 millimeters. A barleycorn is a seed of the barley plant, a common crop in England. Seeds were often used in farming societies as units of length or weight. In medieval England the barleycorn provided a legal basis for both length and weight units: its weight is the grain. barn (b) a slightly humorous unit of area used in nuclear physics. When atoms are bombarded with smaller particles such as electrons, the electrons are scattered as if the nucleus of the atom was a tiny solid object. The barn is used to express the apparent cross-sectional area of this scattering object. One barn is equal to 10-28 square meters. Using this unit, physicists can say that such and such a nucleus is "as big as a barn," or 10 barns, or whatever. The proper SI unit to use for these measurements is the square femtometer (fm2); one barn equals 100 fm2. barrel (bbl) [1] a commercial unit of volume used to measure liquids such as beer and wine. The official U. S. definition of the barrel is 31.5 gallons, which is about 4.211 cubic feet or 119.24 liters. In Britain the barrel is defined to be 36 imperial gallons, which is substantially larger: about 5.780 cubic feet or 163.66 liters. (An older British barrel, equal to 1/2 hogshead, is 26.25 imperial gallons.) There are other official barrels, defined in certain states; most of them fall in the general range of 30-40 gallons. barrel (bbl or bo) [2] a commercial unit of volume used to measure petroleum. By international agreement a barrel of petroleum equals 42 U. S. gallons, which is about 158.987 liters. The symbol bo (barrel of oil) is used for this unit in the petroleum industry. barrel (bbl) [3] a commercial unit of volume used to measure dry commodities such as flour. The U. S. dry barrel is 105 dry quarts, which is about 4.083 cubic feet or 115.63 liters. (This is the only case in the United States customary system where a dry volume is less than the corresponding fluid volume.) barrel (bbl) [4] a commercial unit of weight, varying with the commodity being measured. In the U.S., for example, a barrel of flour traditionally holds 196 pounds, and a barrel of beef, fish, or pork 200 pounds. A barrel of cement is traditionally equal to 4 bags, which is 376 pounds in the U.S. and 350 pounds in Canada. barrel bulk a commercial unit of volume equal to exactly 5 cubic feet or 0.141 584 cubic meters. This is a unit of convenience; there are exactly 8 barrels bulk in a freight ton and 20 in a register ton. barrique a large French wine barrel holding 225 liters (about 59.44 U.S. gallons). barye (ba) the CGS unit of pressure, equal to 1 dyne per square centimeter. Pronounced "bar-ee," the barye derives from the Greek word for weight, barys. It's a very small unit; 1 barye is equal to 0.1 pascal (Pa), 1 microbar (¥ìb), or about 14.5 x 10-6 pound per square inch. basis point (bp) a unit of proportion equal to 0.01% or 10-4. The basis point is used in finance to measure small fluctuations in interest rates and the rates of return on investments. Prior to the introduction of the basis point, these fluctuations were measured clumsily in 64ths of a percent. basis weight a unit used in the paper industry to express the weight (really, the thickness) of paper. The basis weight is the weight in pounds of one ream of a "basic size" sheet. For bond paper, the traditional basic size is 17 x 22 inches. baud (Bd) a unit used in engineering for measuring the rate of data transmission over telegraph or telephone lines. The baud rate is the number of times per second the signal carrying the communication varies in strength or frequency. In the days of the telegraph, the signal increased at the start of a pulse and decreased at the end, so the baud rate was 2 times the number of dots that could be transmitted per second. In recent years, when the baud has been used to represent digital data transmission, the definition has varied according to the technique used. If the signal has only two states (on or off) then the baud rate is the same as the transmission rate in bits per second. Since this was the case in early (slow) modems, in common language the baud is often used to mean the same thing as 1 bit per second. However, faster modems use signals having 4 possible states (corresponding to 2 bits per second) or 16 possible states (corresponding to 4 bits per second). For those of us who are not communications engineers, the best way to represent data transmission rates is directly in bits per second (b/s or bps), avoiding the use of the baud altogether. The baud is named for the French telegraph engineer J. M. E. Baudot (1845-1903), the inventor of the first teleprinter. Baume a measure of relative density; see degree Baume. BC, BCE abbreviations for "before Christ" and "before common era," respectively. These are standard designations for years before the beginning of the common or Christian era (see CE). Oddly, there is no year designated in the common system for numbering years; the year 1 CE (or 1 AD) was preceded by 1 BCE (or 1 BC). The lack of a year means that the number of years elapsing between the year n BCE and the year m CE (or n BC and m AD) is n + m - 1. B/D symbol for barrels of oil per day (see barrel [2] above), a unit used in the energy industry to measure the rate at which oil is pumped from a well. Beaufort scale an empirical scale, first devised by the British admiral Sir Francis Beaufort (1774-1857), for estimating wind speed by observing the effects of the wind. Using the scale, sailors can judge the wind velocity by observing the wind's effects on the waves. There is a corresponding scale for observers on land. For example, a moderate gale (32-38 miles per hour) is described as "force 7" on the Beaufort scale. The Beaufort scale numbers come rather close to being a unit of measurement, because they are equal to the whole number closest to 0.66 times the wind velocity in miles per hour raised to the exponent 2/3 (Beaufort force = 0.66 x (velocity2/3)). becquerel (Bq) the SI fundamental unit of activity, usually meaning radioactivity. "Radioactivity" is caused when atoms disintegrate, ejecting energetic particles. One becquerel is the radiation caused by one disintegration per second. The unit is named for a French physicist, Antoine-Henri Becquerel (1852-1908), the discoverer of radioactivity. See also curie. Note: both the becquerel and the hertz are basically defined as one event per second, yet they measure different things. The hertz is used to measure the rates of events that happen periodically in a fixed and definite cycle. The becquerel is used to measure the rates of events that happen sporadically and unpredictably, not in a definite cycle. bee space an informal unit of distance used in beekeeping. In a hive, bees seal up an opening smaller than a bee space, and they fill a larger opening with new honeycomb. If an opening is equal to a bee space, the bees leave it open as a passageway. A hive can be disassembled to remove the honey if the individual comb frames are carefully spaced one bee space apart. This discovery, made by the British beekeeper Lorenzo Longstroth in 1852, is crucial to modern beekeeping. The exact size of the bee space varies somewhat with the strain of bees being raised, but it is generally very close to 1/4 inch or 6.5 millimeters. bel (B) a unit of sound intensity, invented by engineers of the Bell telephone network in 1923 and named in honor of the inventor of the telephone, Alexander Graham Bell (1847-1922). If one sound is 1 bel louder than another, this means the louder sound is 10 times more intense than the fainter one. A difference of 2 bels corresponds to an increase of 10 x 10 or 100 times in intensity. The beginning of the scale, bels, represents a sound intensity of 1 picowatt per square meter; this is approximately the faintest sound which can be detected by a person who has good hearing. In practice, sound intensity is almost always stated in decibels. One bel is equal to approximately 1.151 293 nepers. bell a traditional unit of time. On ships at sea, a practical measure of time is the watch, a period of 4 hours. The watch is divided into 8 bells, so one bell equals 1/2 hour or 30 minutes. Every 30 minutes the ship's bell sounds the number of bells elapsed since the start of the watch. BeV a symbol for billion electron volts sometimes used in the U.S. The correct symbol is GeV. bhp abbreviation for brake horsepower. The brake horsepower of an engine is the effective power output, sometimes measured as the resistance the engine provides to a brake attached to the output shaft. See horsepower. bicron (¥ì¥ì) an obsolete metric unit of distance, defined as 10-12 meter. Coined after the micron, the bicron has been replaced by the picometer (pm). BID abbreviation for the Latin phrase bis in die, twice a day, a unit of frequency traditionally used by doctors in writing medical prescriptions. biennium a unit of time equal to two years. Many states, including North Carolina, elect legislators every two years and adopt budgets for the two-year period, called the biennium. billet a single stick of firewood. Traditional English billets had a length of 40 inches (1.016 meters). billiard a unit of quantity equal to 1015, which is one quadrillion in American terminology or 1000 billion in traditional British terminology. The name is coined to parallel milliard, which has long been a name for 1000 million. biot (Bi) another name for the abampere, a unit of electric current equal to 10 amperes. This unit honors the French mathematician and physicist Jean-Baptiste Biot (1774-1862), one of the founders of the theory of electromagnetism. bit (b) [1] the basic unit of information. Each bit records one of the two possible answers to a single question: "0" or "1," "yes" or "no," "on" or "off." Logically, this is the smallest quantity of information that can exist. The word "bit", coined by the American statistician and computer scientist John Tukey (b. 1915) in 1946, is an acronym for binary digit. bit (b) [2] a logarithmic unit of storage capacity, equal to the base-2 logarithm of the number of possible states of the storage device or location. If data is stored as binary digits, this reduces to definition [1]: an 8-bit storage location, for example, has 28 = 256 possible states, so its capacity is log2 28 = 8 bits. If, however, a storage location stores one letter, then it has 26 possible states, and its storage capacity is log2 26 = 4.7004 bits. bit (b) [3] a unit of information content, now known properly as the shannon. bit per second (b/s or bps) a common unit of data transmission rate in computer science. The symbol bps is often pronounced "bips." Modem transmission rates are often stated in kilobits per second (Kbps or kb/s). block an informal unit of distance popular in the U.S. A block is the average distance between street intersections in the rectangular street grids common in most American cities. The length of a block varies from about 1/20 mile (80 meters) in New York to about 1/16 mile (100 meters) in many midwestern cities to about 1/10 mile (160 meters) in cities of the South and West. board foot (bd ft, fbm, or BF) a unit of volume used for measuring lumber. One board foot is the volume of a one-foot length of a "standard board" twelve inches wide by one inch thick. Thus a board foot equals 144 cubic inches, or 1/12 of a cubic foot, or approximately 2.360 liters. If lumber is stacked neatly, we can compute the number of board feet from the dimensions of the stack, no matter how wide or thick the boards are. For example, a stack of two-by-fours 4 ft high, 4 ft wide, and 8 ft long contains 4 x 4 x 8 = 128 cubic feet, equivalent to 128 x 12 = 1536 board feet. Board of Trade unit a British name for the kilowatt hour, a unit of electrical energy. body mass index (BMI) a measure of obesity used in medicine and health. The BMI is equal to a person's weight W (in kilograms) divided by the square of his or her height H (in meters): BMI = W/H2. If measurements are made in traditional English units, the equivalent formula is BMI = 703.07¡¤W/H2, where W is measured in pounds and H in inches. In the U.S., a person with a BMI of 25 or more is considered overweight, and a person with a BMI of 30 or more is considered obese. boe abbreviation for "barrel of oil equivalent," a commercial unit of energy. One boe is equal to about 6.119 gigajoules (GJ) or 5.800 million Btu. bohr or bohr radius (a0) a unit of distance used in particle physics. The bohr radius represents the mean distance between the proton and the electron in an unexcited hydrogen atom. It equals about 52.918 picometers (pm), or 52.918 x 10-12 meter. The unit is named for the Danish physicist Niels Bohr (1885-1962), who explained the structure of atoms in a famous paper in 1913. bolt a commercial unit of length or area, used to measure finished cloth. Generally speaking, one bolt represents a strip of cloth 100 yards long, but the width varies according to the fabric. Cotton bolts are traditionally 42 inches wide and wool bolts are usually 60 inches wide. Thus a bolt of cotton is 1160 square yards (969.91 m2) and a bolt of wool is 1660 square yards (1387.97 m2). bone-dry unit (bdu) a unit used in the forest products industry to measure bulk products such as wood chips. One bone-dry unit is a volume of wood chips (or whatever) which would weigh 2400 pounds (1.0886 metric ton) if all the moisture content were removed. The bone-dry ton (bdt) is similar but based on a weight of 2000 pounds (0.9072 metric ton). bore the bore of a gun is the inside diameter of its barrel. As a result, "bore" is sometimes used as a unit, meaning the same as gauge [1] for shotguns and the same as caliber [1] for other guns. bottle (btl) a unit of volume. Like actual bottles, this unit varies according to the nature of the contents. For a long time in the U.S., a bottle of milk was 1 quart (946.36 milliliters) and a bottle of whiskey was 1 fifth (757.1 milliliters). Today wine is customarily sold in bottles containing 750 milliliters (about 25.37 U.S. fluid ounces or 26.40 British Imperial fluid ounces). See also quart [2]. Other countries have traditional units of about this size. For example, the Russian boutylka contains 768.95 milliliters. BP abbreviation for "years before present," a unit of time used in anthropology, geology, and paleontology. bpd abbreviation for barrels per day, a unit of production used in the petroleum industry. braca see braza, below. braccio a traditional Italian unit of distance. The word means "arm", and a braccio is the length of a man's arm, about 27 or 28 inches (68-71 centimeters). In modern times, the braccio has become a metric unit of exactly 70 centimeters (about 27.56 inches). brace another name for a pair. The word is used mostly by hunters, who may speak of a brace of partridges or a brace of shotguns. Derived from the Latin word bracchia for both arms, it literally means "one for each arm." braza a traditional unit of distance in Spain and Latin America. The braza is comparable to the English fathom. In Spain it equals 2 varas, 8 palmos, or about 1.67 meters (5.48 feet or 65.75 inches). In Latin America the braza tends to be larger. The Argentine braza is 1.73 meters (5.68 feet or 68.16 inches), and under the Texas definition of the vara the braza would be 1.693 meters (5.556 feet or 66.67 inches). The Portuguese braca is similar, but it equals 10 palmos or about 2.20 meters (7.22 feet or 86.6 inches). breadth another name for a span (9 inches). breakfast cup a unit of liquid volume, used in food recipes in Britain. The breakfast cup corresponds to the cup used by American cooks, except that it is based on British Imperial units. Thus it equals 1/2 Imperial pint. This is equivalent to 10 Imperial fluid ounces, 17.339 cubic inches, 1.20 U.S. cup, or about 284 milliliters. This unit is also called a tumblerful. bright, brightness a unit describing surface brightness, or reflectivity, especially of paper. The fraction of light reflected by a surface is called its albedo, and its brightness is the albedo expressed as a percentage. Thus 88 bright paper reflects 0.88, or 88%, of the light falling on the paper. bril a unit used to express the "brilliance" or subjective brightness of a source of light. The scale used is logarithmic: an increase of 1 bril means doubling the luminance (and thus the actual amount of light energy) emitted by the source. A luminance of 1 lambert (or 10 000 lux) is defined to have a brilliance of 100 brils. Mathematically, the brilliance in brils equals (log L)/log 2 + 100, where L is the luminance in lamberts. Brinell hardness (Bhn) a measure of the hardness of a metal introduced by J. A. Brinell in 1900. In the Brinell test, a hard object such as a steel ball is pressed into the material being tested. The ball is of a specified diameter, usually 1 centimeter. The Brinell hardness is the amount of force applied to the ball divided by the area of the indentation the ball makes in the material. The result is usually stated in kilograms of force per square millimeter. British thermal unit (Btu) a unit of heat energy defined as the amount of heat required to raise the temperature of one pound of water by one degree Fahrenheit. In America the British thermal unit is sometimes called the heat unit. In defining the Btu, it is necessary to specify the temperature of the water; thus there have been several definitions over the years. However, one Btu is equal to about 251.996 (small) calories, or 0.251 996 of the (kilo-)calories counted by dieters. Using the current definition of the calorie (the IT calorie), one Btu equals approximately 778.169 foot pounds, 1.055 056 kilojoules or 0.293 071 watt hour. brix a unit of proportion, equal to percent, used by vintners (winemakers) in measuring the concentration of sugar in unfermented grape juice. Thus one brix equals 1%. The unit is named for the Austrian scientist Adolf Brix (d. 1870), who invented a hydrometer that reads directly the percentage of sugar in the juice, provided the reading is taken at a specified temperature. bucket a unit of volume, generally informal. In the U.S., many commodities (both wet and dry) are sold in plastic buckets holding 5 U.S. liquid gallons (about 18.927 liters). In Britain, a bucket is often understood to be 4 gallons, or 18.182 liters based on the British Imperial gallon. bunder a traditional Dutch unit of land area. Since the adoption of the metric system in the Netherlands in 1809 the bunder has been considered equal to the hectare (2.471 acres). Historically, the unit varied with locality, generally in the range 0.85-1.3 hectare. The unit was also used in Belgium, being called the bonnier in French-speaking provinces. bundle (bdl) [1] a unit of quantity for paper, equal to 2 reams or 40 quires. This would be 960 sheets using the old definition of 24 sheets per quire, or 1000 sheets using the newer quire of 25 sheets. bundle (bdl) [2] a traditional unit of length for yarn, equal to 20 hanks. For cotton yarn, a bundle contains 16 800 yards (about 15.362 kilometers). For wool, a bundle contains 11 200 yards (10.241 kilometers). bundle (bdl) [3] in the construction trades, a bundle is a package of shingles. Shingles are usually packed so that exactly 3, 4, or 5 bundles are needed to cover a square (100 square feet). Asphalt shingles are often sold in bundles of 27, with 3 bundles per square, but the heavier cedar shingles generally require 5 bundles per square. bushel (bu) [1] a traditional unit of volume used for measuring dry commodities such as grains and fruits. In the United States, the customary bushel is based on an old British unit known as the Winchester bushel. This unit dates to the early fourteenth century, at least: King Edward I defined the bushel to be 8 gallons in 1303. The form used in the U.S. was legalized by Parliament in 1696. One U.S. bushel equals 4 pecks, 8 gallons, or 32 (dry) quarts; this is a volume of 2150.42 cubic inches or about 1.2445 cubic feet, and represents the volume of a cylindrical container 18.5 inches (47.0 cm) in diameter and 8 inches (20.3 cm) deep. The U. S. bushel holds about 35.239 07 liters. Traditionally, there is also a heaped bushel, which is 27.8% (sometimes 25%) larger than a regular bushel. The regular bushel is also called struck measure to indicate that the bushels have been struck, or leveled, rather then heaped. The origin of the word bushel is unclear; some scholars believe it derives from an ancient Celtic unit, but most believe it is of medieval French origin. bushel (bu) [2] a unit of volume in the British imperial system (see gallon [3]) equal to 8 imperial gallons, or 2219.36 cubic inches (1.2844 cubic feet). The British bushel holds about 36.369 liters. bushel (bu) [3] a commercial unit of weight for grains and other bulk commodities. Agricultural commodities such as wheat are traditionally sold by the bushel, but because commodities tend to settle and compact in shipping, disputes over the volume delivered arise easily. To avoid these disputes, traders in a market or a country generally agree on a standard weight for one bushel of the commodity. Often this standard weight is set by law. Although the bill of lading still shows "bushels," it is really the weight rather than the volume which is sold and guaranteed. For example, in the United States a bushel of wheat equals 60 pounds (27.216 kg), a bushel of barley 48 pounds (21.772 kg), a bushel of oats 32 pounds (14.515 kg), and a bushel of rye 56 pounds (25.401 kg). A more complete table is included. butt a traditional unit of volume used for wines and other alcoholic beverages. A butt is generally defined to be two hogsheads, but the size of hogsheads varies according to the contents. In the United States a hogshead is typically 63 gallons and a butt is 126 gallons: about 16.844 cubic feet or 476.96 liters. In Britain, a butt of beer is 108 imperial gallons: about 17.339 cubic feet or 490.98 liters. The word comes from the Roman buttis, a large cask for wine. button measure a unit of distance equal to 1/40 inch (0.635 millimeter), used, as you might guess, for measuring the thickness of buttons. This unit is also called the line, a bad idea since the line is generally equal to 1/12 inch. byte (B) a unit of information used in computer engineering. One byte is generally equal to 8 bits, which means that a byte has 28 = 256 possible states. This enough to store, for example, one character of text. The unit was named by IBM engineer Werner Buchholz in 1956, and its 8-bit size was standardized shortly thereafter by IBM's System 360, a top-selling early mainframe computer. The spelling "byte" is used instead of "bite" in order to avoid confusion with the bit. C c a unit of velocity equal to the speed of light. One of the fundamental principles of physics is that light always travels at the same speed in a vacuum, exactly 299 792 458 meters per second or about 670 617 300 miles per hour. Another fundamental principle is that nothing can travel faster than light. At speeds which are large fractions of the speed of light, the theory of relativity predicts a variety of strange physical effects. In calculations involving relativity, speeds are customarily expressed as fractions of the speed of light, such 0.95 c. C the Roman numeral 100, sometimes used as a unit of quantity or as a prefix meaning 100, as in Cwt (hundredweight) or CCF (100 cubic feet). CA, CCA abbreviations for "cranking amps" and "cold cranking amps," respectively. These units are often seen on motor vehicle batteries in the U.S. The amps involved are ordinary amperes of electric current. "Cranking amps" measure the current supplied by the battery when starting the vehicle at a temperature of 32 ¡ÆF (0 ¡ÆC), while "cold cranking amps" measure the current supplied at ¡ÆF (-17.8 ¡ÆC). cable a unit of distance formerly used at sea. The traditional mariner's cable was 120 fathoms long. This is equal to 720 feet, or 0.1185 nautical mile, or about 219.4 meters. A different version of the cable, used in the British Navy, was equal to exactly 0.1 nautical (Admiralty) mile, which is 608 feet or about 185.3 meters. caliber (cal) [1] a unit used to express the diameter of the bore of a gun. (The bore is the inside diameter of the gun barrel.) Traditionally, the diameter was stated in inches, so that ".22 caliber" referred to a pistol having a bore of 0.22 inches (5.588 mm). This usage is declining, because bore diameters of many guns are now stated directly in millimeters. "Caliber" is the American spelling; elsewhere the unit is spelled "calibre." caliber (cal) [2] for naval guns, caliber is defined as the length of the gun barrel divided by the diameter of the bore. Thus a "50-caliber" gun on a warship has a barrel 50 times longer than its bore. calorie (cal) the CGS unit of heat energy. This calorie (also called a gram calorie or small calorie) is the amount of heat required at a pressure of one atmosphere to raise the temperature of one gram of water by one degree Celsius. Unfortunately, this varies with the temperature of the water, so it is necessary to specify which degree Celsius is meant. A traditional choice was the degree from 14.5¡ÆC to 15.5¡ÆC; raising the temperature of water through this range requires 4.1858 joules, a quantity called the 15¡Æ calorie. Another choice produces the thermochemical calorie, equal to exactly 4.184 joules. More common today is the international steam table calorie, or IT calorie for short, defined by an international conference in 1956 to equal exactly 4.1868 joule or about 0.003 968 32 British thermal units (Btu). The name of the unit comes from the Latin calor, heat. Calorie (Cal or kcal) the MKS unit of heat energy (also called the kilogram calorie or large calorie). This unit is often (but not always!) distinguished from the small calorie by capitalizing its name and symbol. The large calorie is the amount of heat required at a pressure of one atmosphere to raise the temperature of one kilogram of water by one degree Celsius. Since this is 1000 times as much water as mentioned in the definition of the small calorie, the large calorie equals 1000 small calories, 4.1868 kilojoules, or 3.9683 Btu. These are the calories that joggers are trying to get rid of, the ones we gain by eating. The use of the same term "calorie" for two different-size units is endlessly confusing, but we seem to be stuck with it. candela (cd) the fundamental SI unit for measuring the intensity of light. Candela is the Latin word for "candle." The unit has a long and complicated history. Originally, it represented the intensity of an actual candle, assumed to be burning whale tallow at a specified rate in grains per hour. Later this definition was replaced with a definition in terms of the light produced by the filament of an incandescent light bulb. Still later a standard was adopted which defined the candela as the intensity of 1/600 000 square meter of a "black body" (a perfect radiator of energy) at the temperature of freezing platinum (2042 K) and a pressure of 1 atmosphere. This definition has also been discarded, and the candela is now defined to be the luminous intensity of a light source producing single-frequency light at a frequency of 540 terahertz (THz) with a power of 1/683 watt per steradian, or 18.3988 milliwatts over a complete sphere centered at the light source. The frequency of 540 THz corresponds to a wave length of approximately 555.17 nanometers (nm). Light of this frequency has a yellow-green color and roughly the same visual brightness as ordinary daylight. In addition, normal human eyes are more sensitive to this wavelength than to any other. In order to produce 1 candela of single-frequency light of wavelength l, a lamp would have to radiate 1/(683V(l)) watts per steradian, where V(l) is the relative sensitivity of the eye at wavelength l. Values of V(l) are defined by the International Commission on Illumination (CIE). candle (cd) an older name of the unit now known officially as the candela (see above), or for the candlepower (see below). candlepower (cp) a unit formerly used for measuring the light radiating capacity of a lamp or other light source. One candlepower represents the radiating capacity of a light with the intensity of one "international candle," or about 0.981 candela as now defined. Since 1948 the candela has been the official SI unit of light intensity, and the term "candlepower" is no longer in much use. candy a traditional weight unit of South Asia. The candy was quite variable, generally within the range 500 to 800 pounds (225 to 365 kilograms). In the international cotton trade, the candy was generally equal to exactly 7 (British) hundredweight, which is 784 pounds or 355.62 kilograms. cantar an English spelling for the Arab form of the quintal. In recent years, the cantar has been interpreted as a metric unit equal to 50 kilograms (110.23 pounds); traditional cantars tended to be a few percent larger than this. carat (ct or c) [1] a unit of mass used for diamonds and other precious stones. Some say the word carat comes from the Arabic qirrat, meaning the seed of a coral tree, and some say it comes from the Greek keration, a carob bean. Perhaps both seeds were used as standards for weighing precious stones. Traditionally the carat was equal to 4 grains. The definition of the grain differed from one country to another, but typically it was about 50 milligrams and thus the carat was about 200 milligrams. In the U. S. and Britian, the diamond carat was formerly defined by law to be 3.2 troy grains, which is about 207 milligrams. Jewelers everywhere now use a metric carat defined in 1907 to be exactly 200 milligrams. carat (ct or c) [2] in Britain, the spelling "carat" is used for the unit of gold purity known in America as the karat. category the ranking of a hurricane on the Saffir-Simpson scale. catty a weight unit of the colonial period in East and Southeast Asia, originating as the kati in Malaya. The catty varied a little from market to market. Typically it was equal to about 1 1/3 pounds avoirdupois (604.79 grams), and it is still equal to that weight in Malaysia. In Thailand, the catty is used now as a metric unit equal to exactly 600 grams (1.3228 pounds). In China, the catty was identified with the jin, a traditional Chinese unit. cb- abbreviation for "cubic," seen in combinations such as cbm (cubic meter) or cbft (cubic foot). The proper symbol for cubic meters is now m3, not cbm. cc an alternate symbol for the cubic centimeter (cm3). The cubic centimeter represents the same volume as the milliliter, a third alternative for measuring small volumes. CCF an abbreviation for 100 cubic feet. Local water and sewer utilities often sell water in CCF units; for this purpose one CCF equals about 748.05 gallons (U.S.) or about 2831.7 liters. Utilities sometimes sell natural gas in CCF units; for this purpose the CCF is really a unit of energy equivalent (roughly) to the therm. CE abbreviation for "common era." This abbreviation, a non-religious designation for years of the common (or Christian) era, is used in place of the traditional AD. Years of the common era are supposed to be counted from the birth of Jesus of Nazareth, founder of the Christian religion. However, the present numbering system was not established until more than 500 years later. It is based on calculations of the monk Dionysius Exiguus placing Jesus's birth in the Roman year 753 AUC. (This is definitely wrong; the Biblical account of Jesus's birth implies that he was born several years before the death of Herod the Great, who died, we now know, in 4 BCE. Thus the calculation of the common era is off by 6 or 7 years at least, and the third millenium actually started several years ago!). In the conventional use of the common era system, there is no year and the year prior to 1 CE is designated 1 BCE. In astronomy, however, it simplifies calculations to use a year CE (= 1 BCE) and apply negative numbers to earlier years. Thus Herod died in -3 CE. Celsius see degree Celsius. The word "degree" is often omitted in informal statements of temperature, as in "we expect a high of about 23 Celsius today." Celsius heat unit (Chu) a unit of heat energy, equal to the energy required to raise the temperature of one pound of water by 1¡ÆC at standard atmospheric pressure. 1 Chu is equal to exactly 1.8 Btu, approximately 453.59 IT calories (see above), or 1.8991 kilojoules. The unit is also called the centigrade heat unit. cent a unit used in music when it is necessary to specify the ratio in frequency between two tones with great precision. There are 100 cents in a semitone, or 1200 in an octave. If two notes differ by 1 cent, the ratio between their frequencies is 21/1200 or approximately 1.0005778. cental (cH or ctl) an alternate name in Britain for the U.S. hundredweight, which is equal to exactly 100 pounds (the British hundredweight is 112 pounds). Introduced by British merchants around 1850, the name was apparently coined after the model of the quintal. The cental has sometimes been confused with the centner (see below). centi- (c-) a metric prefix meaning one hundredth, or 0.01. The prefix comes from the Latin word centum for one hundred. centiare (ca) a metric unit of area. The centiare equals 0.01 are, which just happens to be the same as the square meter (about 10.7639 square feet). centigrade a temperature scale; see degree centigrade. centigram (cg) a metric unit of mass equal to 10 milligrams or about 0.154 grain. centihg an informal unit of pressure equal to 1 centimeter of mercury. This is equivalent to 10 millimeters of mercury, approximately 0.3937 inHg, 0.1933 lb/in2, 13.33 millibars, or 1333 pascals. The word is pronounced "sentig". centiliter (cl or cL) a common metric unit of volume. One centiliter equals 10 cubic centimeters; this is about 0.610 24 cubic inch, 0.3318 U.S.fluid ounce or 0.3519 British fluid ounce. In the kitchen, a centiliter is roughly equal to 2 U.S. teaspoons (or 0.704 British tablespoonful). centimeter (cm) [1] the fundamental unit of distance in the CGS metric system, equal to 0.01 meter. One centimeter is about 0.393 700 787 inch. centimeter (cm) [2] an obsolete name for the statfarad (approximately 1.11 picofarad), the CGS electrostatic unit of capacitance. centimeter (cm) [3] an obsolete name for the abhenry, the CGS electromagnetic unit of inductance. The abhenry is the same as the nanohenry. centimeter of mercury (cmHg) a traditional unit of pressure equal to 10 mmHg, 1.333 22 kilopascal, or about 0.193 pounds per square inch. centimeter of water (cmH2O or cm WC) a unit of pressure equal to the pressure exerted at the Earth's surface by a column of water 1 centimeter high. This is about 98.067 pascals, 0.980 67 millibars, 0.3937 inch of water, or 2.04 pounds per square foot. The unit is used in respiratory medicine and elsewhere to measure air pressures. centimorgan (cM) a unit of genetic separation used in genetics and biotechnology. If two locations on a chromosome have a 1% probability of being separated during recombination in a single generation, then the distance between those locations is one centimorgan. In humans, the centimorgan is approximately equal to one million base pairs. The unit honors the pioneering American geneticist Thomas Hunt Morgan (1866-1945), who received the 1933 Nobel Prize in Medicine for his discoveries concerning the role played by the chromosome in heredity. centner the English name for a German weight or mass unit, the zentner. The name centner should not be used for the cental (see above). centrad a unit of angle measure equal to 0.01 radian or about 0.572958¡Æ (34'22.65"): a "centiradian," in other words. century a traditional unit of time equal to 100 years. cetane number a measure of the ability of diesel fuel to reduce engine knocking. The cetane number plays the same role in diesel engine technology that the octane number plays in conventional automobile engine technology. It is the percentage by volume of cetane which must be added to methylnaphthalene to give the mixture the same resistance to knocking as the diesel fuel sample being tested. cfm, cfs abbreviations for cubic feet per minute and cubic feet per second, respectively; these are English customary units of flow. 1 cfm = 28.317 liters per minute and 1 cfs = 28.317 liters per second. CFU abbreviation for "colony forming units," a count of the number of active bacterial cells in preparations of Lactobacillus acidophilus and other "friendly" organisms of the digestive system. Counts as high as one billion CFU per gram are not uncommon. chain (ch) a unit of distance formerly used by surveyors. The traditional British surveyor's chain, also called Gunter's chain because it was introduced by the English surveyor Edmund Gunter in 1620, is 4 rods long: that's equal to exactly 1/10 furlong, 22 yards, or 66 feet (20.1168 meters). The chain was divided into 100 links. American surveyors sometimes used a longer chain of 100 feet, known as Ramsden's chain. chalder or chaldron (chd) a traditional British unit of volume or weight, used for dry commodities such as coal or lime. As a volume measure, the chaldron is equal to 36 bushels, or 288 British Imperial gallons; this is equivalent to 46.237 cubic feet or 1.3091 cubic meters. As a measure for coal, the chalder equals 1/8 keel or 53 hundredweight (5936 pounds or 2692.52 kilograms). The words "chalder," "chaldron," and "cauldron" are English spellings of the same old French word, which originally meant a large kettle. champagne quart see quart [2]. character (char) a unit of information used in computer science and telecommunications. One character is usually equal to 8 bits or one byte. There is, however, a new coding system designed to accommodate all the characters used in all the world's languages; this system, Unicode, assigns 16 bits or 2 bytes to each character code. charka a traditional Russian unit of volume containing about 123.0 milliliters, 4.159 U.S. fluid ounces or 4.329 imperial fluid ounces. There are 6.25 charki in a boutylka (bottle) and 10 in a schtoff. The word charka means a cup or glass. cheval vapeur (cv) French name for the metric horsepower. ch'ih a unit of distance used in China during the colonial period. The ch'ih equals 10 t'sun, 35.814 centimeters, or 14.1 inches. There are 1800 ch'ih in a li. chilo- Italian version of the metric prefix kilo- (1000). chin one of several spellings in English for the jin, a traditional Chinese weight unit. choppin a traditional Scottish unit of volume equal to 2 mutchkins or 1/2 Scots pint. The choppin is equivalent to about 51.702 cubic inches, 0.8953 U.S. liquid quart, 1.491 British imperial pints, or 0.8473 liters. cicero a unit of distance used by typesetters and printers in continental Europe, equal to 12 Didot points. This is approximately 0.1780 inch or 4.52 millimeters. The cicero corresponds to the British and American pica. circle (cir) the traditional unit of angle measure, divided into 2 pi radians or 360 degrees. circular inch, circular mil informal units of area. A circular inch [mil] is the area of a circle one inch [mil] in diameter. Since the area of a circle is proportional to the square of the diameter, a circle of diameter X inches [mils] has an area of X2 circular inches [mils]. The circular inch is equal to approximately 0.785 398 square inches or 5.067 07 square centimeters. The circular mil is equal to approximately 0.785 398 square mils or 506.707 square micrometers. Clark degree see degree [4]. clausius a unit of entropy. Entropy is a measure of the extent to which heat or energy in a physical system is not available for performing work. It is computed in units of energy per kelvin. One clausius is equal to 1 Calorie per kelvin (Cal/K) or 4.1868 kilojoules per kelvin (kJ/K). The unit honors the German physicist Rudolf Clausius (1822-1888), who introduced and named the concept of entropy in 1850. click U.S. military slang for the kilometer (about 0.621 mile). Also spelled klick or klik. This unit was widely used during the Vietnam War, but it must have originated earlier. The Random House Dictionary of American Slang reports it was used by U.S. troops in Germany during the 1950s. It may have been used during the Korean War (1950-53). Veterans are welcome to let me know their recollections. Occasionally it was used as a non-metric unit equal to 1000 yards (0.9144 kilometer). clo a unit of thermal insulation used for clothing. One clo is intended to represent the insulation required to keep a resting person warm in an indoor room at 70¡ÆF (21.1¡ÆC). The rate of a person's heat loss is measured in watts per square meter of skin area per kelvin of temperature difference across the clothing; the value of insulation is measured by the reciprocal of this rate, in square meter kelvins per watt (m2K/W). One clo is equal to 0.155 m2K/W or 1.550 togs. clove an old English unit of weight. A clove is usually considered equal to 1/2 stone; that's 7 pounds (3.175 kilograms) by the modern definition of the stone, but in the past the clove varied from 6.5 to 8 pounds. coffee measure a flat-bottomed scoop or spoon used to measure coffee beans for grinding in U.S. homes. The coffee measure holds 2 U.S. tablespoons (about 29.57 milliliters). coffeespoon a unit of volume formerly used in U.S. food recipes. A coffeespoon is 1/2 teaspoon, 1/12 fluid ounce, or about 2.5 milliliters. column inch (col in) a unit of relative area used in journalism. A column inch is an area one column wide and one inch deep; it is often used to measure the length of newspaper stories. cong a metric unit of area used in Vietnam. One cong equals 1000 square meters, which is 0.1 hectare, 0.24177 acres, or 1196.00 square yards. coomb or coom a traditional British unit of volume used mostly for dry commodities. A coomb is 4 imperial bushels; this is equivalent to 5.1374 cubic feet or about 145.48 liters. cord (cd) [1] a traditional unit of volume used to measure stacked firewood. Like most traditional units of trade, the cord has varied somewhat according to local custom. In the United States, the cord is defined legally as the volume of a stack of firewood 4 feet wide, 8 feet long, and 4 feet high. That's a volume of 128 cubic feet, about 3.6247 cubic meters, or 3.6247 steres. The name apparently comes from an old method of measuring a stack of firewood using a cord or string. cord (cd) [2] in the timber industry, the cord is also used as a unit of weight for pulpwood. The weight varies with tree species, ranging from about 5200 pounds (2.36 metric tons) for pine to about 5800 pounds (2.63 metric tons) for hardwood. cord foot (cd ft) a traditional unit of volume used to measure stacked firewood. A cord foot is the volume of a stack of firewood 4 feet wide, 1 foot long, and 4 feet high. Thus the cord foot is 1/8 cord, or 16 cubic feet, or about 0.4531 cubic meter. coulomb (C) the SI unit of electric charge. One coulomb is the amount of charge accumulated in one second by a current of one ampere. Electricity is actually a flow of particles called electrons, and one coulomb represents the charge on approximately 6.241 506 x 1018 electrons. The coulomb is named for a French physicist, Charles-Augustin de Coulomb (1736-1806), who was the first to measure accurately the forces exerted between electric charges. count (ct) [1] a unit of quantity equal to 1. This unit is used in commerce to specify that the quantity stated represents a reliable count. For example, a carton marked "oranges 24 ct" contains exactly 24 oranges. count (ct) [2] a traditional unit measuring the texture of a fabric, equal to the number of threads per inch. A 100 count fabric has 39.37 threads per centimeter. count (ct) [3] an informal unit of volume in bartending. Bartenders usually fit bottles with pourers designed to restrict the flow to 0.5 fluid ounce per second (14. 8 mL/s). They can them measure a quantity of liquid by counting, "one thousand one, one thousand two, ..." while pouring. This is much faster than using a measuring glass, and just about as accurate. cran a traditional unit of volume formerly used by fishermen. The cran, originally intended to represent the volume of fish held by a barrel, was standardized at 37.5 Imperial gallons, or about 170.46 liters. This is somewhat larger than the usual Imperial barrel. crith a unit of mass sometimes used in the physics and chemistry of gases. The crith is equal to the mass of a liter of hydrogen at standard temperature (0.01¡ÆC) and pressure (1 atmosphere); this is about 89.885 milligrams. crore a traditional unit of quantity in India, equal to 107 or 10 million. The word is an English transliteration of a Hindi word. cu abbreviation for "cubic," as in cubic feet or cubic meters. This symbol is forbidden by the SI, but it remains in common use in ordinary English text. The proper symbol for cubic feet is now ft3, not cu. ft. cuartillo a traditional Spanish unit of volume comparable to the liter or the English quart. The cuartillo equals 4 octavillos or 1/4 almude and contains 1.156 25 liters, which is about 1.222 U.S. liquid quart or 1.017 British imperial quart. cubic centimeter (cm3 or cc) the CGS unit of volume, equal to 10-6 cubic meter, 1 milliliter, or about 0.061 023 7 cubic inch. cubic foot (ft3, cu ft, or cf) a traditional unit of volume in English speaking countries. One cubic foot equals 1728 in3, 1/27 yd3, 0.028 316 85 m3, or 28.316 85 liters. The cubic foot also holds about 7.4805 U.S. gallons or about 6.2288 British Imperial gallons. cubic inch (in3 or cu in) a traditional unit of volume in English speaking countries. One cubic inch equals 1/1728 = 5.787 037 x 10-4 ft3, 16.3871 cm3, 16.3871 milliliters, 0.5541 U.S. fluid ounce, or 0.5767 British Imperial fluid ounce. cubic meter (m3) the SI unit of volume, equal to 106 cm3, 1000 liters, 35.3147 ft3, or 1.307 95 yd3. A cubic meter holds about 264.17 U.S. gallons or 219.99 British Imperial gallons. cubic yard (yd3) a traditional unit of volume in English speaking countries. One cubic yard equals 27 ft3, 46 656 in3, 0.764 555 m3, or 764.555 liters. A cubic yard holds about 201.97 U.S. gallons or about 168.20 British Imperial gallons. cubit a historic unit of distance frequently mentioned in the Bible. The word comes from the Latin cubitum, "elbow," because the unit represents the length of a man's forearm from his elbow to the tip of his outstretched middle finger. This distance tends to be about 18 inches or roughly 45 centimeters. In ancient times, the cubit was usually defined to equal 24 digits or 6 palms. The Egyptian royal cubit, however, was equal to 28 digits or 7 palms. In the English system, the digit is conventionally identified as 3/4 inch; this makes the ordinary cubit exactly 18 inches (45.72 centimeters). The Roman cubit was shorter, about 44.4 centimeters (17.5 inches). The ordinary Egyptian cubit was about the same length as the Roman cubit, and the royal cubit was about 51.8 centimeters (20.4 inches). cuerda (cda) [1] a traditional unit of land area in Puerto Rico. The cuerda is equal to about 3930 square meters, 4700 square yards, 0.393 hectare, or 0.971 acre. Because the cuerda and the acre are so close to being equal, they are often treated informally as being equal. cuerda (cda) [2] a traditional unit of distance in Guatemala, equal to 25 varas or about 21 meters (roughly 69 feet). Since cuerda means a cord or rope in Spanish, this unit probably arose as the length of a measuring rope. The cuerda is also used as an area measure equal to 1 square cuerda or 625 square varas; this is about 440 square meters or 527 square yards. cuerda (cda) [3] a traditional unit of volume for firewood in Cuba, analogous to the U.S. cord (see above). A cuerda of firewood is equal to 128 cubic pies, 2.87 cubic meters, or 0.79 cord. cumec an informal unit of flow equal to 1 cubic meter per second or about 35.3147 cubic feet per second. The name was coined after the model of the cusec (see below). cunit a measure of wood volume used in forestry. One cunit (pronounced cue-nit) is a volume of timber containing 100 cubic feet (2.8317 cubic meters) of actual wood (excluding bark and air between the logs). The unit is used mostly for wood intended as pulpwood or firewood. cup (c) a traditional unit of volume used in recipes in the United States. One cup equals 1/2 pint, or 8 fluid ounces. Technically, one cup equals exactly 14.4375 cubic inches or approximately 236.6 milliliters, not that anyone measures quite so precisely in the kitchen. American cooks use the same size cup for measuring both liquid and dry substances. In Britain, cooks sometimes use a similar unit called the breakfast cup. curie (Ci) a unit of radioactivity. One curie was originally defined as the radioactivity of one gram of pure radium. In 1953 scientists agreed that the curie would represent exactly 3.7 x 1010 atomic disintegrations per second, or 37 gigabecquerels (GBq), this being the best estimate of the activity of a gram of radium. See also becquerel. The unit is named for Pierre and Marie Curie, the discoverers of radium. cusec a traditional unit of flow equal to 1 cubic foot per second or about 28.317 liters per second. cut a traditional unit of length for yarn in Scotland and northern England. One cut equals 1/12 hank, a unit varying with the material. A cut of cotton yarn is 70 yards; a cut of wool is 46 2/3 yards. Cwt traditional symbol for hundredweight. cycle (c) an informal name for "cycle per second." The frequencies of radio signals and of alternating electric current were previously stated in cycles; thus the current in American homes is often described as "60-cycle" and a radio station might describe its signal frequency as "1040 kilocycles", really meaning 1040 kilocycles per second. cycles per second (cps) a traditional unit of frequency equal to one per second, or one hertz. Almost all measurements of frequency are now stated in hertz, the SI unit. D dag symbol for the dekagram (10 grams; see entry below), a common unit of mass in metric countries. daily value (DV) a unit of nutrition used in the United States. The federal government establishes recommended daily amounts of various nutrients, both "good" ones like vitamins and "bad" ones like fat and sodium. These so-called daily values are based on a hypothetical person, male or female, who requires a diet of 2000 Calories per day. The results are approximate at best, since nutritional needs vary with age, sex, and other factors. Food packages generally carry nutritional lables specifying the amount of each nutrient contained in a standard serving, expressed as a percentage of the daily value (%DV). Link: official daily values for each nutrient, from the U.S. Food and Drug Administration. dal symbol for the dekaliter (10 liters), a common metric unit of volume. dalton (Da or D) an alternate name for the atomic mass unit (u or amu). The dalton is often used in microbiology and biochemistry to state the masses of large organic molecules; these measurements are typically in kilodaltons (kDa). It seems almost necessary to have such a unit, since "kilo-amu" would be such a clumsy name. The unit honors the English chemist John Dalton (1766-1844). dan a traditional Chinese weight unit, previously spelled tan in most English works. During the European colonial era the unit was equal to 100 catties or 133.333 pounds. In modern China the dan is equal to 100 jin, which is exactly 50 kilograms (110.231 pounds). darcy a CGS unit of permeability. Permeability is the extent to which a solid allows the flow of a fluid. This flow depends on the properties of the solid and also on the dynamic viscosity of the fluid and the difference in pressure driving the flow. One darcy is the permeability of a solid through which one cubic centimeter of fluid, having a viscosity of one centipoise, will flow in one second through a section one centimeter thick and one square centimeter in cross section, if the pressure difference between the two sides of the solid is one atmosphere. It turns out that permeability has the same units as area; since there is no SI unit of permeability, square meters are used. One darcy is equal to about 0.98692 x 10-12 square meter. The unit is named for a French scientist, H. Darcy (1803-1858), who did pioneering work in the study of permeability. dash [1] an informal unit of volume used in food recipes. American cookbook editors usually figure 2 dashes to the pinch and 8 pinches to the teaspoon. This makes the dash equal to roughly 0.01 fluid ounce or 0.3 milliliter. dash [2] an informal unit of volume used in bartending. The bartender's dash (or splash) is usually taken to be 1/32 fluid ounce or about 0.92 milliliter. day (d or da) a traditional unit of time marked by the rotation of the Earth on its axis. As viewed from outside the Solar System, Earth turns on its axis once in approximately 23 hours 56 minutes 4 seconds, a period astronomers call the sidereal day. The day seems a little longer than that to us, because by the time 23:56:04 has passed, the Earth has moved 1/365 of the way in its orbit around the Sun. This causes the Sun to appear a little farther east than it did at the start of the day. It takes the Earth, on the average, about 3 minutes 56 seconds to turn a bit more and put the Sun back in the same apparent position it occupied at the start of the day. There's more. The speed of the Earth in its orbit varies, because the Earth moves faster when it is closer to the Sun. This causes the length of an actual day to vary slightly over the course of the year. (Despite what you might think, days are actually longer during the Northern Hemisphere winter and shorter during the Northern Hemisphere summer, because, again despite what you might think, the Earth is actually closer to the Sun during the Northern Hemisphere winter.) Because of these irregularities in the length of actual days, we adjust our clocks to measure the average period of time between two successive noons, that is, between two successive appearances of the Sun on the meridian (the imaginary line joining due north to due south across the sky). Astronomers call this period of time the mean solar day. By ancient custom we divide the day into 24 hours, or 1440 minutes, or 86 400 seconds. But there is also a small problem with doing this. The second has its own scientific definition based on the frequency of a certain radiation from cesium atoms and having nothing to do with the Earth's rotation. The reason for making this scientific definition of the second is that the length of the mean solar day is not constant from one year to the next. The gravitational attraction of the Moon is very gradually slowing the Earth's rotation, so that each day is a tiny bit longer, about 40 nanoseconds on the average, than the previous day. This lengthening adds one second to the length of the day about every 60 000 years. During the first decades of the 21st century the actual length of the mean solar day will be about 86 400.002 seconds. For this reason, time as kept by our best clocks runs a bit faster than time as kept by the Earth. To keep everything in step, every so often a leap second is added at the end of a day. This is done either on June 30 or on December 31. Because the rate of slowing is erratic, the need for these leap seconds cannot be predicted more than a few months in advance. Link: explanation of leap seconds from the U.S. Naval Observatory. dB- a prefix indicating that a measurement is made using a logarithmic scale similar to that of the decibel (see below) in that a difference of 10 dB- corresponds to a factor of 10. In each case, the actual measurement a is compared to a fixed reference level r and the "decibel" value is defined to be 10 log10(a/r). Many units of this kind have been used and only a few of the more common ones are mentioned in the next entries. dBA, dBC units of sound intensity, exactly like the decibel except that before the measurement is made sounds of high and low frequencies, heard poorly or not at all by the human ear, have been filtered out. The letters A and C refer to two filtering methods. dBc a unit of signal strength used in electronics, especially in measuring noise levels. The signals are measured relative to the strength of the carrier signal, which is the desired signal. A typical statement might be that a certain noise level is -50 dBc, meaning that the noise is 50 "decibels below carrier" or 10-5 times the carrier signal strength. dBFS abbreviation for "decibels full scale," a unit of power as measured by a digital device. A digital measurement has a maximum value M depending on the number of bits used. If the actual power measurement is p, the dBFS value displayed is 20¡¤log10(p/M) dBFS. Since p cannot exceed M, this reading is always negative. dBi a unit measuring the gain of an antenna. The reference level is the strength of the signal that would be transmitted by an isotropic antenna: one radiating equally in all directions. For example, an antenna rated 20 dBi transmits a signal in the desired direction 102 = 100 times stronger than an isotropic antenna. dBm, dBW logarithmic units of power used in electronics. These units measure power in decibels above the reference level of 1 milliwatt in the case of dBm and 1 watt in the case of dBW. A power of n watts equals 10 log n dBW; conversely, a power of p dBW equals 10(p/10) watts. The same formulas link dBm to milliwatts. An increase of 10 dBm or 10 dBW represents a 10-fold increase in power. Since 1 watt = 1000 milliwatts, 1 dBW = 30 dBm. dBrn a symbol for "decibels above reference noise," a unit measuring noise levels in telecommunications. The usual reference level is -90 dBm, which is equivalent to a power of 1 picowatt (1 pW). For example, 50 dBrn equals -40 dBm. dBspl a logarithmic unit of sound intensity as computed from the sound pressure level. The reference level is a pressure of 20 micropascals. If sound waves exert a pressure of P pascals, the sound intensity is 100 + 20¡¤log10(P/2) dBspl. dBu a logarithmic unit of power, similar to dBm but computed from voltage measurements. The reference level is 0.775 volts, the voltage which generates a power of 1 milliwatt across a circuit having an impedance of 600 ohms. A voltage of V volts coresponds to a power of 20¡¤log10(V/0.775) dBu. dBV a logarithmic unit of power, similar to dBm but computed from voltage measurements. The reference level is 1 volt. A voltage of V volts coresponds to a power of 20¡¤log10(V) dBV. dBW see dBm above. dBZ a unit of radar reflectivity used in meteorology. The unit measures the amount of energy returned to a weather radar site as a function of the amount transmitted. The scale is logarithmic, a difference of 10 dBZ indicating a 10-fold increase in energy returned. Generally rain returns a signal of at least 15 dBZ, while the heavy rain and hail of a severe thunderstorm may return 50-60 dBZ. The radar images displayed on television are actually computer-generated plots of dBZ values. deadweight ton (dwt) a traditional unit of weight or mass used in the shipping industry. The deadweight tonnage of a ship is the difference between its weight when completely empty and its weight when fully loaded. This includes the weight of everything portable carried by the ship: the cargo, fuel, supplies, crew, and passengers. The deadweight ton is traditionally equal to the British ("long") ton of 2240 pounds (1016.047 kilograms). However, more and more often it is being taken to equal the metric ton (exactly 1000 kilograms, or 2204.623 pounds). debye (D) a CGS unit of electric dipole moment used in chemistry and physics. A charged molecule can be regarded as a tiny bar with opposite charges at the ends. Such a bar is called a dipole, and its dipole moment is the amount of the charge multiplied by half the length of the bar. Thus the appropriate unit of dipole moment in the SI would be the coulomb meter (C¡¤m). Since this is much too large for molecules, the debye is defined as an electric dipole moment of 10-18 statcoulomb centimeter or 3.33564 x 10-30 coulomb meter. The unit honors the Dutch physicist P.J.W. Debye (1884-1966), who was famous for his research on polar molecules. deca- (da-) a metric prefix meaning 10. The prefix is taken from the Latin word for ten, decem. The Greek form deka- is also used, and it has the advantage of avoiding confusion with deci-, one tenth. The International Bureau of Weights and Measures (BIPM) specifies deca- for the prefix and da- for the symbol. In its interpretation of the International System (SI) for U.S. use, the U.S. National Institute of Standards and Technology (NIST) recommends deka- for the spelling of the prefix. The SI allows national variations in the spelling of units and prefixes, but not in symbols. Following NIST recommendations, listings for units beginning deca- will be found below under deka-. decade [1] a traditional unit of time equal to 10 years. decade [2] a unit of time equal to 10 days or 1/3 month. The revolutionary govenments in France (1790s) and the Soviet Union (1920s) tried to decimalize and secularize the calendar by eliminating the week (with its traditional day of religious observance) and substituting the decade. Both efforts failed. However, the decade survived as a unit of civil time in Russia and as a 10-day unit in some scientific contexts (for example, a 10-day cycle in meteorology is called a decade). Where the decade equals 1/3 month, the three decades of a month are the 1st-10th, 11th-20th, and 21st through the last day of the month. decade [3] another name for the bel or the order of magnitude: a logarithmic unit used to compare the sizes of quantities. Two quantities differ by one decade if one is 10 times the other, by two decades if one is 10¡¤10 = 100 times the other, and so on. decatherm see dekatherm, below. decay time a unit of relative time used in physics. Decay time is similar to half life, but shorter and less familiar. It is the time required for an exponentially decaying process (such as radioactivity) to decrease to 1/e = 36.7879% of its original value. The fraction of activity remaining at time T, if T is measured in decay time units, is simply e-T. The decay time equals 0.693 147 half life. deci- (d-) a metric prefix meaning one tenth, or 0.1. The prefix is derived from the Latin word decimus for a tenth. decibel (dB) the customary unit for measuring sound intensity. The human ear is capable of detecting an enormous range of sound intensities. Furthermore, our perception is not linear. Experiment shows that when humans perceive one sound to be twice as loud as another, in fact the louder sound is about ten times as intense as the fainter one. For this reason, sound intensity is measured on a logarithmic scale. Specifically, if one sound is 1 bel (10 decibels) louder than another, this means the louder sound is 10 times more intense than the fainter one. A difference of 20 decibels corresponds to an increase of 10 x 10 or 100 times in intensity. The beginning of the scale, decibels, represents a sound intensity of 1 picowatt per square meter; this is approximately the faintest sound which can be detected by a person who has good hearing. Sound waves having an intensity of decibels exert an air pressure of about 20 micropascals (¥ìPa) RMS. A quiet room has a normal sound intensity of around 40 decibels, ten thousand times louder than the faintest perceptible sound, and a thunderclap may have an intensity of 120 decibels, a trillion times louder than the faintest sound. One decibel equals about 0.115 129 neper. decigram (dg) a metric unit of mass, equal to 100 milligrams or about 1.5432 grains. deciliter (dl or dL) a common metric unit of volume, equal to 0.1 liter or 100 cubic centimeters. A deciliter contains 6.10237 cubic inches, 3.38140 U.S. fluid ounces, or 3.519 British fluid ounces. The deciliter is similar in size to the gill, an old English unit of volume. decimeter (dm) a fairly common metric unit of distance, equal to 10 centimeters or 3.9370 inches. The decimeter is very close to the hand, a traditional English unit. degree (¡Æ or deg) [1] the standard unit of angle measure, equal to 1/360 circle, 60 minutes, 3600 seconds, or about 0.017 453 293 radian. degree (¡Æ or deg) [2] a unit of distance sometimes used at sea, equal to 60 nautical miles, approximately 69.05 statute miles, or 111.12 kilometers. This distance is the average length of one degree of latitude. On the Earth's surface, one degree of latitude actually varies from about 68.7 miles at the Equator to 69.4 miles at the poles, while one degree of longitude is about 69.17 miles at the Equator (shrinking to nothing at the poles!). degree (¡Æ or deg) [3] any of the various units of temperature (see below). In the U.S., the unmodified unit "degree" generally means the degree Fahrenheit; in other countries, it means the degree Celsius. degree (¡Æ or deg) [4] a unit measuring the hardness of water. Water is called "hard" if it contains a high concentration of mineral salts, especially calcium carbonate. This concentration can be expressed clearly in parts per million (ppm) or in milligrams per liter (mg/L). Starting in the mid-19th century, several uses of "degree" for water hardness became well established. In the U.S. and Britain, hardness is measured in Clark degrees, named for the scientist who devised the first reliable test for water hardness. The Clark degree is defined as 1 part of calcium carbonate per 70 000 parts of water; this is about 14.3 ppm, 17.1 mg/L, or 1 grain per gallon (gpg), another popular measure of hardness. The French, properly decimal as always, used a degree equal to exactly 10 ppm, while the German degree was equivalent to 17.8 ppm. degree (¡Æ or deg) [5] various scientific quantities, especially specific gravity and viscosity, have been measured in "degrees" based on the readings of particular instruments. The degree scales of Baume (see below) and Beck, and many others, have been used in measuring specific gravity, and the scales of Engler and MacMichael are common in the measurement of viscosity. Often these degree scales can't be converted in a simple way to the quantities being measured; one needs the conversion formula or table for the particular instrument. Some of this data can be found in scientific and engineering handbooks, especially older ones. degree (¡Æ or deg) [6] the percentage of alcohol, by volume, present in a mixture. In winemaking, for example, a 13¡Æ wine is 13% alcohol by volume (13% v/v). This unit is also called the degree Gay-Lussac (¡Æ GL) after the French chemist Joseph-Louis Gay-Lussac (1778-1850). degree (deg) [7] a unit used in mathematics to describe algebraic expressions. The degree of an expression having a single variable is the highest exponent (or power) to which that variable appears. If the expression has more than one variable, then the degree is the highest total exponent of the individual terms, where in each term the exponents of the different variables are added. Thus x2y4z3 is a term of degree 9. degree (¡Æ) [8] the DIN speed rating for film; see DIN below. degree API (¡ÆAPI) a unit invented by the American Petroleum Institute (API) to measure the specific gravity, or one might say the "lightness," of petroleum. Lighter oils are more valuable, so the API scale gives them higher readings than heavier oils. If S is the specific gravity of the petroleum, the API degree rating is equal to (141.5/S) - 131.5 degrees. degree Balling see degree Plato, below. degree Baume (¡ÆB) a unit of relative density, as read on a type of hydrometer invented by the French chemist Antoine Baume (1728-1804). Two scales are used, depending on whether the liquid is lighter than water or heavier than water. For lighter liquids, the relative density d in degrees Baume is related to specific gravity S by the formula d = (144.3 / S) - 144.3; for heavier liquids the formula is d = 144.3 - (144.3 / S). degree Celsius (¡ÆC) the metric unit of temperature. The Celsius temperature scale is named for its inventor, the Swedish astronomer and physicist Anders Celsius (1701-1744). It was designed so that the freezing point of water (at one atmosphere of pressure) is ¡ÆC, while the boiling point is 100 ¡ÆC. Thus the Celsius degree is defined to be 1/100 of the temperature difference between these two temperatures. In the SI system, the temperature scale has been redefined so that the temperature of the triple point of water (the temperature at which water can exist simultaneously in the gaseous, liquid, and solid states) is exactly 0.01 ¡ÆC, and the size of the degree is 1/273.16 of the difference between this temperature and absolute zero (the temperature at which all molecular motion ceases). For practical purposes, however, there is no difference between this and the original definition. See also degree Fahrenheit, kelvin. degree centigrade (¡ÆC) an older name for the degree Celsius. The Celsius scale is called "centigrade" because it has 100 (centi-) gradations between the freezing point and boiling point of water. About 1850 scientists began calling the unit "degree Celsius" in order to honor its inventor. degree day (deg da) a unit used in meteorology and engineering to measure the demand for heating or cooling. In the United States, it's agreed that 65 ¡ÆF is the desirable temperature for the inside of a building. The number of degree days recorded on a given date is equal to the difference between 65 and the mean temperature for that date (figured as the average of the highest and lowest temperatures). If the mean temperature is above 65, cooling degree days are recorded; if it is below 65, heating degree days are recorded. The total number of heating or cooling degree days recorded in any period of time measures rather accurately the demand during that time for fuel used in heating and cooling buildings. degree Fahrenheit (¡ÆF) a traditional unit of temperature still used customarily in the United States. The unit was defined by the German physicist Daniel G. Fahrenheit (1686-1736), who also invented the mercury thermometer. Fahrenheit set 0¡Æ at the coldest temperature he could conveniently achieve, and he intended to set 100¡Æ at the temperature of the human body. (He was off a little there; normal temperature for humans is between 98 ¡ÆF and 99 ¡ÆF.) On this scale, the freezing point of water (at normal sea level atmospheric pressure) turned out to be about 32 ¡ÆF and the boiling point about 212 ¡ÆF. Eventually the scale was precisely defined by these two temperatures. 1¡ÆF equals 5/9 ¡ÆC, but in converting between scales we have to be careful to adjust the zero points as well. To convert a temperature in ¡ÆF to the Celsius scale, we must first subtract 32¡Æ and then multiply by 5/9. In the other direction, to convert a temperature in ¡ÆC to the Fahrenheit scale, we must first multiply by 9/5 and then add 32¡Æ. The Celsius scale is now used everywhere outside the United States, so only Americans need to remember these formulas. degree Gay-Lussac (¡ÆGL) see degree [6] above. degree Kelvin (¡ÆK) see kelvin. The SI requires that temperatures on the absolute temperature scale be stated in kelvins, not in degrees Kelvin. degree KMW (¡ÆKMW) a unit used in Austria to measure the sugar content of must, the unfermented liquour from which wine is made. One degree KMW is roughly equivalent to 1% sugar by weight or 5¡Æ Oe; for the exact conversion see the next entry. KMW is an abbreviation for Klosterneuburger Mostwaage (must scale). degree Oechsle (¡ÆOe) a unit used in Germany and Switzerland to measure the sugar content of must, the unfermented liquour from which wine is made. One degree Oechsle (or Ochsle) is roughly equivalent to 0.2% sugar by weight. This unit is related legally to ¡ÆKMW by the formula ¡ÆOe = ¡ÆKMW * ([.022 * ¡ÆKMW] + 4.54). degree Plato (¡ÆP) a unit measuring sugar content, especially of the wort, the unfermented liquour from which beer is made. Named for a German chemist, one degree Plato represents a sugar content equivalent to 1% sucrose by weight. Not all the sugar in a wort is sucrose; the unit standardizes the measurement to the sucrose equivalent. The reading is made with a device called a saccharometer. The degree Balling is a somewhat older unit equivalent (approximately) to the degree Plato. degree Rankine (¡ÆR) a unit of absolute temperature, now replaced by the kelvin. 1 ¡ÆRankine represents the same temperature difference as 1 ¡ÆFahrenheit, but the zero point of the scale is set at absolute zero. This means the Rankine temperature is 459.67¡Æ plus the Fahrenheit temperature. 1 ¡ÆRankine is equal to exactly 5/9 kelvin. The unit is named for the British physicist and engineer William Rankine (1820-1870). degree Reaumur (¡Ær) a unit of temperature formerly used in continental Europe. The Reaumur temperature scale, named for the French scientist Rene-Antoine Ferchault de Reaumur (1683-1757), is similar to the Celsius scale, but the interval between the freezing and boiling temperatures of water equals 80 ¡Ær instead of 100 ¡ÆC. Therefore, 1 ¡Ær equals 1.25 ¡ÆC or 4/9 ¡ÆF. deka- (da-) a metric prefix meaning 10, taken directly from the Greek word for ten, deka. The Latin spelling deca- is also used (see deca-, above). Although the U.S. National Institute for Standards and Technology recommends deka-, deca- is seen frequently in American publications. The spelling deca- is also used in French and in other languages where "k" rarely occurs in the middle of a word. Regardless of spelling, da- is the official symbol. (The symbol dk- for deka- is incorrect, but it is seen fairly often.) dekagram or decagram (dag) a common metric unit of mass, the dekagram is frequently used in European food recipes. One dekagram is equal to 10 grams, 0.01 kilogram, or 0.352 739 66 ounce. The symbol dkg sometimes used for this unit is incorrect. dekaliter or decaliter (daL or dal) a common metric unit of volume equal to 10 liters and comparable to the English peck. The dekaliter is equal to about 2.641 72 U.S. liquid gallons, 2.270 21 U.S. dry gallons (1.135 10 U.S. pecks), or 2.199 69 British imperial gallon (1.099 85 British pecks). The symbol dkL sometimes used for this unit is incorrect. dekameter or decameter (dam) a common metric unit of distance equal to 10 meters (about 32.8084 feet). The symbol dkm sometimes used for this unit is incorrect. dekan a unit of angle measure equal to 10¡Æ or 1/36 circle. The ancient Egyptians divided the circle of the Zodiac into 36 divisions, which the Greeks called dekans. The unit is still used occasionally in astrology, where one dekan equals 1/3 sign. dekatherm or decatherm a unit of energy equal to 10 therms, 1 million Btu, or about 1.055 057 gigajoules (GJ). This unit is used in the energy industry as a synonym for the million-Btu (MM Btu). denaro a traditional Italian weight unit equal to 24 grani; its size varied from about 1.1 grams to 1.25 grams. denier [1] a traditional unit of yarn density. One denier is the density of a thread having a mass of 1 gram per 9 kilometers of length. The metric unit of yarn density is the tex; 1 denier equals 1/9 tex. denier [2] a traditional French weight unit, comparable to the English pennyweight. Like the pennyweight, the denier is equal to 24 grains (1.275 grams). This unit was also called the scrupule. dessertspoon or dessertspoonful (dsp) a unit of volume sometimes used in food recipes. The dessertspoon is equal to 2 teaspoons; this is roughly equivalent to 10 milliliters in the U.S. and 7 milliliters in Britain. dessiatina a traditional unit of land area in Russia. By coincidence, the dessiatina is very nearly the same as a hectare: it equals 1.0924 hectare or 2.6997 acres. diameter (dia) a unit of magnification, equivalent to power. "Power" tends to be used in adjective phrases, such as "120-power eyepiece" or "7-power binoculars." "Diameters" often appear in declarative statements of magnification, such as, "the eyepiece lens magnifies 120 diameters." diamond a mysterious marking on many tape measures in the U.S. The diamonds mark a distance unit equal to exactly 8/5 feet (19.2 inches or 48.768 centimeters). This is useful to carpenters if they wish to place 5 studs, floor joists, etc., in a distance of 8 feet. dicker a traditional unit of quantity equal to 10. The word is an English version of the Latin word for ten, decem. After being used for centuries, especially by traders in furs and skins, the dicker has nearly disappeared from the English language. Didot point a unit of distance used in typography; see point [2]. digit a historic unit of distance equal to the width of a person's finger. The digit was used in all the ancient civilizations of the Middle East and Mediterranean, and it was equal to 0.75 inch or 19 millimeters with only the smallest variations. Typically, there were 4 digits in a palm, 16 in a foot, and 24 (sometimes 28) in a cubit. The word digit is from the Latin word for a finger or toe, digitus. dimension (dim) a mathematical unit measuring the number of independent directions in a set or space. Traditionally, a space has as many dimensions as there are mutually perpendicular directions at each point in the space: thus a line has 1 dimensions, a plane has 2 dimensions, and the ordinary space we live in has 3 dimensions. The theory of relativity is set in a "space-time" having 4 dimensions, and higher dimensional spaces are frequently used in science and economics. In addition, mathematicians have developed several methods for assigning fractional dimensions to certain complex sets. DIN the initials of Deutsches Institut fur Normung, the German standards agency. Outside the U.S., film was generally marked with a DIN speed rating such as DIN 24 (corresponding to ASA 200). A difference of 3 in the DIN rating corresponds to a doubling of the film speed; that is, DIN 27 film (ASA 400) is twice as fast as DIN 24. The ASA and DIN ratings are now combined, with a degree symbol on the DIN number, and marked with ISO (the initials of the International Standards Organization). Thus DIN 27 film is now marked ISO 400/27¡Æ. diopter (dpt or D) a metric unit used in optics to measure the refractive power of a lens. Each lens has a focal length, defined to be the distance from the center of the lens to the point at which the lens focuses light. The shorter the focal length, the greater the refractive power of the lens. The refractive power of the lens, in diopters, equals 1 divided by the focal length of the lens, in meters, so 1 diopter = 1 m-1. The unit is often spelled dioptre outside the United States. The lower case Greek letter delta is also used as a symbol for the diopter. dk- an incorrect symbol for the metric prefix deka- (10), sometimes seen in combinations such as dkg for the dekagram or dkm for the dekameter. The correct symbol is da-. Dobson unit (DU) a unit used in geophysics to measure the ozone in the atmosphere. One Dobson unit represents the amount of atmospheric ozone that would form a uniform layer 0.01 millimeter (10 micrometers) thick at standard temperature (0 ¡ÆC) and pressure (1 atmosphere or 1013.25 millibars). Under normal conditions, the atmosphere contains about 300 Dobson units of ozone, but this falls to 100 Dobson units or less in the "ozone holes" over the Earth's poles. The unit is named for the British physicist G.M.B. Dobson; in 1920 he invented a spectrometer to measure ozone concentrations from the ground. donkey power a somewhat light-hearted metric unit of power equal to 250 watts or about 0.3353 horsepower. donum see dunum (below). doppelzentner (Dz) a name often used in Germany for the metric quintal. Since a zentner is 50 kilograms, a doppelzentner or "double" zentner equals 100 kilograms (about 220.462 pounds). douzieme a traditional unit of distance used in watchmaking. The word is French for "twelfth," and a douzieme is equal to 1/12 Swiss ligne. This is about 188 micrometers (microns) or 7.4 mils. dozen (doz or dz) a familiar unit of quantity, equal to 12. Division into units of 12 rather than 10 has the advantage that 12 can be evenly divided into halves, thirds, or quarters. For this reason, units of 12 have been common since the earliest civilizations of the Middle East. "Dozen" comes from an old French word dozaine related to the Latin word duodecem, "twelve." dpi a unit used to measure the resolution, or sharpness, of a photograph or video image. The unit, an abbreviation for "dots per inch," remains in use even though individual picture elements are now called pixels rather than dots. dpm, dps abbreviations for decay per minute and decay per second, units used in the measurement of rates of radioactivity. 1 dps is properly called 1 becquerel (Bq), and 1 dpm = 1/60 Bq. drachm traditional British spelling of dram (see below). drachma a traditional unit of weight in Greece and the eastern Mediterranean. The drachma was a Greek coin, and the unit was originally the weight of the coin. This is about 3.2 grams, 49.3 grains, or 0.113 ounce avoirdupois. There is also a traditional Dutch weight unit of the same name, but it is somewhat larger: 3.906 grams, 60.28 grains, or 0.1374 ounce avoirdupois. Compared to a similar English unit, the Greek drachma is about 2.5 pennyweight and the Dutch is almost exactly 3 pennyweight. drachme a traditional unit of weight in France and Germany, generally between 3.7 and 3.9 grams. dram (dr) [1] a unit of weight in the traditional avoirdupois system (see pound [1]), equal to 1/16 ounce or 1/256 pound. One dram equals about 1.7718 gram. The word dram comes from a Latin weight unit, the dragma, and derives ultimately from the Greek drachme, meaning a handful. The word is usually spelt "drachm" in Britain and "dram" in the United States, but both spellings are pronounced "dram." dram (dr) [2] a unit of weight in the traditional system of English apothecaries (see scruple), equal to 60 grains, 1/8 troy ounce, or approximately 3.8879 gram. The apothecaries' dram is sometimes abbreviated dr ap to distinguish it from the avoirdupois dram. It is equivalent to about 2.1943 avoirdupois drams. There is a similar Italian unit, the dramma, equal to 72 grani or roughly 3.5 grams. dram (dr) [3] a traditional unit of volume. See fluid dram. drex a unit used in the textile industry to measure the density of a single fiber of yarn. One drex equals a density of one gram per 10 kilometers of length, or 1 microgram per centimeter (¥ìg/cm). One drex equals 0.1 tex. drink a unit measuring the alcohol content of beverages, used in describing the medical effects of alcohol. U.S. physicians generally consider one drink equal to 0.5 U.S. fluid ounces of alcohol; the appropriate metric equivalent would be 15 milliliters. In U.S. fluid units, one drink corresponds to about 4 ounces of wine or 1.25 ounces of whiskey. drop (gtt) [1] a unit of volume used in pharmacy. Traditionally the drop was another name for a minim, a unit of volume equal to 1/60 fluid dram or 1/480 fluid ounce (about 0.0616 milliliter in the U.S., 0.0592 milliliter in Britain). Now that prescriptions are written in metric units, the pharmacist's drop is equal to exactly 0.05 milliliter. The abbreviation, traditional in pharmacy, is from the Latin gutta, drop. drop [2] a informal unit of volume used in recipes. According to some older kitchen references, 24 drops = 1/4 teaspoon; with U.S. definitions this makes the drop equal to 1/576 fluid ounce or about 0.51 milliliter, comparable to the pharmacist's drop (previous entry). drop [3] a traditional unit of weight in Scotland, equal to 1/16 troy ounce or about 1.944 grams. drum a unit of volume sometimes used in the oil trade. A standard drum of oil contains 55 U.S. gallons or about 208.198 liters. One drum equals 1.3095 barrels [2]. dunum or donum a traditional unit of land area in the Middle East. The unit is of Turkish origin, but it seems to be obsolete in modern Turkey. As it is commonly used in Israel and Jordan, the dunum is a metric unit equal to 1000 square meters or 0.1 hectare (about 0.2471 acre). dwt abbreviation for deadweight ton (see above); also the traditional symbol for pennyweight. dyad a unit of quantity equal to 2. This unit is frequently used in cell biology and biotechnology. dyne (dyn) the CGS unit of force. One dyne is the force that accelerates a mass of one gram at the rate of one centimeter per second per second. Expressed in SI units, the dyne equals 10-5 newton. This is quite a small force: it equals about 2.248 x 10-6 pound of force (lbf) in the traditional English system. The word dyne comes from the Greek dynamis, power. E e [1] a unit of electric charge equal to the charge on one electron. The charges on other particles in atomic physics are whole-number multiples of this charge. 1 e is equal to approximately 160.217 764 62 x 10-21 coulomb, or 160.217 764 62 zeptocoulombs (zC). e [2] a mathematical unit used as the base of "natural" logarithms and exponentials. The real number e is irrational, which means that its decimal expansion is infinite and non-repeating. To 25 significant digits e equals 2.718 281 828 459 045 235 360 287. Of the many properties of this number, the most important is that the rate of change in the function ex is equal to the value of the function itself. As a result, the larger the value of this function is, the faster the function grows: the behavior we call "exponential growth." The Swiss mathematician Leonard Euler (1707-1783) introduced the symbol e, probably because it is the first letter of the word "exponential." Other mathematicians continued to use the letter in his honor, sometimes calling it Euler's constant. eclipse year a unit of time used in astronomy in the prediction of eclipses. The plane of the moon's orbit around the earth and the plane of the earth's orbit around the sun intersect in a line called the line of nodes. Eclipses of the sun or moon can only occur at times when the earth, moon, and sun all lie very close to this line. However, the line of nodes rotates, so comparable appearances of the sun on the line do not fall exactly a year apart. The actual interval between these crossings, the eclipse year, has a length of 346.62 days. effective dose (ED) a measure used in pharmacology to express the percentage of a population which receives the desired benefit from a dose of the substance being studied. The measurement is often given as a subscript. For example, the ED50 dose is the amount of the substance which benefits 50% of the test population. el the Dutch ell (see below) a traditional unit of length equal to about 68-70 centimeters (roughly 27 inches). electron (me) a unit of mass used in particle physics, equal to the mass of the electron. This is about 0.910 940 x 10-30 kilogram, 0.910 940 x 10-27 gram, or 0.510 999 MeV (see next two entries). electronvolt (eV) [1] a unit of work or energy used in atomic physics. One electron volt is the work required to move an electron through a potential difference of one volt. The size of the electronvolt must be determined experimentally; the currently accepted value (1998) is 160.217 646 2 x 10-21 joule, or 1.602 176 462 x 10-12 erg. This unit is accepted for use with SI units. The official spelling is electronvolt (one word) rather than electron volt. electronvolt (eV) [2] a unit of mass used in particle physics. Mass and energy are related by Einstein's famous equation, E = mc2. The constant c is the speed of light, 299.79 x 106 m/sec. An energy of 1 electron volt is therefore equivalent to a mass of about 1.7827 x 10-33 gram, or about 1.0736 x 10-9 atomic mass unit. This is such a small unit that most particle masses are measured in MeV or GeV. ell an old English unit of length, used primarily for measuring cloth. One ell equals 20 nails, 45 inches, or 1.25 yards; in metric terms, an ell equals exactly 1.143 meters. The word comes from the Latin ulna, which originally meant the elbow and is now the name of the bone on the outside of the forearm. Probably the unit originated through a custom of measuring lengths of cloth using two forearms, with the hands touching or overlapping. The ell was used with a similar length in France (where it was called the aune). In Scotland, the ell was equal to 37 Scots inches or 37.2 English inches (94.5 centimeters). In eastern Europe, the ell was a shorter distance: see next entry. elle a traditional unit of distance in German speaking countries. The elle varied considerably, but it was always shorter than the English ell or French aune. A typical value in northern Germany was exactly 2 fussen (German feet), which would be close to 24 inches or 60 centimeters. In the south, the elle was usually longer, about 2.5 fussen. In Vienna, the elle was eventually standardized at 30.68 inches (77.93 centimeters). The word elle is often translated "yard" in English, but this is not a very good equivalent. em a printer's unit of relative distance. One em is the height of the point size being used. If 12 point type is being set, then one em is 12 points, and so on. See point [2]. emu an abbreviation for "electromagnetic unit." See ab-. en a printer's unit of relative distance, equal to 1/2 em. If 12 point type is being set, then one en is 6 points. enzyme unit (U) a unit used by biochemists to measure the activity of enzymes, which are proteins produced by living cells to cause or facilitate necessary chemical reactions within the cell. One enzyme unit is the quantity of enzyme needed to cause a reaction to process 1 micromole of substance per minute under specified conditions. eon alternate spelling for aeon, a unit of time equal to 1 billion years. Eotvos unit (E) a unit used in geophysics to measure the change in the acceleration of gravity with horizontal distance. One Eotvos unit equals 10-9 Gal per centimeter or 10-4 Gal per kilometer. In proper SI terms, the Eotvos unit equals 10-9 s-2. The unit honors the Hungarian physicist Roland von Eotvos (1848-1919). ephah a ancient unit of volume for grains and dry commodities, used in the Bible. The ephah is believed to equal about 40.32 liters or 1.4239 cubic feet. This is equivalent to about 1.144 U.S. bushel. epoch a measure of time used in astronomy. In an epoch system, times are specified as years and fractions of years (such as epoch 1998.5). To set a starting point for the system, a specific epoch time must be fixed as a particular clock time of a particular date. In 1984, the International Astronomical Union agreed that epoch times should be fixed by requiring that epoch 2000.0 equals 12 hours Universal Time of the day 2000 January 1. Other epoch conventions were used in the past. See also Julian epoch. The name comes from a Greek word epoche meaning a stopping point or fixed point. equivalent (Eq) a unit of relative amount of substance used in chemistry. "Equivalent" is short for "equivalent weight." One equivalent weight of an element, compound, or ion is the weight in grams of that substance which would react with or replace one gram of hydrogen. Since one gram of hydrogen is very nearly equal to one mole and since hydrogen has one electron free to react with other substances, 1 Eq of a substance is effectively equal to one mole divided by the valence of the substance (the number of electrons the substance would engage in participating in the reaction). In practice, this is a large unit and measurements are more likely to be in milliequivalents (mEq or meq). erg the unit of work or energy in the CGS system, equal to the work done by a force of one dyne acting through a distance of one centimeter. Equivalently, one erg is the kinetic energy of a mass of one gram moving at a velocity of 1 cm/sec. This is equal to 0.1 microjoule, or about 7.375 x 10-8 foot-pound. The name of the unit is from the Greek word ergon, work. erlang (E) a unit of telecommunications traffic. The traffic in erlangs is equal to the number of calls multiplied by the average length of a call. The unit honors A. K. Erlang (1878-1929), a Danish mathematician who studied the mathematics of telephone networks. estadio a traditional unit of distance in Spain and Portugal. The estadio, like the English furlong, is equal to 1/8 mile. The Spanish estadio is equal to 1/8 milla or 625 pies; this is about 571 feet or 174 meters. The Portuguese unit is 1/8 milha, which is much longer: about 856 feet or 261 meters. esu an abbreviation for "electrostatic unit." See stat-. ett- or etto- Italian spelling for the metric prefix hecto- (100). The hectare, for example, is ettaro in Italian. The International System allows national variations in spelling of this kind. etto an informal Italian name for the hectogram, a unit of mass equal to 100 grams or about 3.5274 ounces. exa- (E-) a metric prefix denoting 1018 (one quintillion). The Latin and Greek prefix ex- means "out of," and an exameter (105.7 light years) would definitely take us out of this world. exbi- (Ei-) a binary prefix meaning 260 = 1 152 921 504 606 846 976. This prefix, adopted by the International Electrotechnical Commission in 1998, replaces exa- for binary applications in computer science. The prefix is a contraction of "exabinary." exposure value (EV) a unit used in photography to describe relative exposure. EV is assigned to a specific combination of exposure time and lens aperture, such as 1 second at f/1. The difference between two exposure values is equal to the number of stops separating the two exposure settings. Different combinations of exposure time and lens aperture have the same exposure value; the unit was invented to simplify the relationship. Regardless of camera settings, EV 6 is one stop "faster" than EV 5, that is, an EV 6 setting exposes the film to half as much light as an EV 5 setting. eyesight a term often used with a Snellen fraction in phrases such as "20/20 eyesight." F f or f/ see f ratio, below. F see farad or Fujita-Pearson scale, below. face cord a unit of volume for firewood. See rick. faden an alternate name for the klafter, a German unit of distance. faggot a traditional unit of volume for firewood. A faggot was 3 feet in length and 2 feet in circumference; this is a volume of about 0.955 cubic feet or 27 liters. There are about 134 faggots in a cord. Fahrenheit see degree Fahrenheit. fall [1] a traditional unit of distance equal to 6 ells or 22.5 feet (6.858 meters). This unit was used in land measurement somewhat like the rod. Measurements in rods were often made with an actual wood pole. Measurements in falls were often made with a rope 6 ells long, and the distance falling under the rope was called a fall. fall [2] a traditional unit of area equal to one square fall [1]. A fall of land is 506.25 square feet, 56.25 square yards, or about 47.03 square meters. fanega a Spanish unit of volume, mostly for dry goods. The word is derived from an Arabic word faniqa for a large sack. The fanega equals 12 almudes or 48 cuartillos; this is about 55.50 liters or 1.960 cubic feet (1.575 U.S. bushels). Very similar units were used in Portugal and in most of the Latin American countries. In Chile and Argentina, however, a much larger fanega, roughly 2.5 U.S. bushels, was customary. farad (F) the SI unit of electric capacitance. Very early in the study of electricity scientists discovered that a pair of conductors separated by an insulator can store a much larger charge than an isolated conductor can store. The better the insulator, the larger the charge that the conductors can hold. This property of a circuit is called capacitance, and it is measured in farads. One farad is defined as the ability to store one coulomb of charge per volt of potential difference between the two conductors. This is a natural definition, but the unit it defines is very large. In practical circuits, capacitance is often measured in microfarads, nanofarads, or sometimes even in picofarads (trillionths of a farad). The unit is named for the British physicist Michael Faraday (1791-1867), who was known for his work in electricity and electrochemistry. faraday (Fd) a unit of electric charge. In a process called electrolysis, chemists separate the components of a dissolved chemical compound by passing an electric current through the compound. The components are deposited at the electrodes, where the current enters or leaves the solution. The British electrochemist and physicist Michael Faraday (1791-1867) determined that the same amount of charge is needed to deposit one mole of any element or ion of valence one (meaning that each molecule of the ion has either one too many or one too few electrons). This amount of charge, equal to about 96.4853 kilocoulombs, became known as Faraday's constant. Later, it was adopted as a convenient unit for measuring the charges used in electrolysis. One faraday is equal to the product of Avogadro's number (see mole) and the charge (1 e) on a single electron. fathom (fth or fath) a traditional unit of distance, equal to 2 yards or 6 feet (approximately 1.829 meters). The word comes from the Old English f©¡thm, meaning "outstretched arms", because a fathom is the distance between a man's outstretched fingertips. This is a generic unit which has been used in many cultures; other versions include the Spanish braza, the French toise, the German klafter, the Danish favn (6.18 feet or 1.88 meters), and the Japanese ken. fatt a traditional unit of volume for grain, generally equal to 9 bushels or 1/4 chalder. This would be about 317 liters based on the traditional grain bushel now used in the U.S. or about 327 liters based on the British imperial bushel. fbm a symbol sometimes used for the board foot. The "bm" stands for "board measure." feddan an Egyptian unit of land area, formerly used throughout the Middle East and North Africa. The feddan equals about 0.42 hectare or 1.038 acre. femto- (f-) a metric prefix standing for 10-15 (one quadrillionth). The prefix was coined from femten, which means fifteen in Danish and Norwegian. fermi (f) a metric unit of distance used in atomic physics. One fermi equals 10-15 meter, or 1 femtometer. The unit is named for Enrico Fermi (1901-1954), the Italian-American physicist who built the first nuclear reactor. There was a need for this unit before the metric prefixes for very small quantities were defined; now distances formerly measured in fermis are stated in femtometers instead. FEU a unit of cargo capacity, especially for container ships. These ships carry cargo in standard metal boxes, called containers, which can be transferred easily to trains or trucks. FEU is an abbreviation for "forty-foot equivalent unit." One FEU represents the cargo capacity of a standard container 40 feet long, 8 feet wide, and (usually) a little over 8 feet high. One FEU equals about 25 register tons (see ton [3]) or 72 cubic meters. fifth [1] a traditional U.S. unit of liquid volume equal to 1/5 gallon. The fifth contains exactly 46.2 cubic inches, or about 757.1 milliliters. This unit is really the U.S. version of the traditional bottle. fifth [2] a unit used in music to describe the ratio in frequency between notes. Two notes differ by a fifth if the higher note has frequency exactly 3/2 times the frequency of the lower one. On the standard 12-tone scale, the perfect fifth is approximated as 7 half steps, corresponding to a frequency ratio of 27/12 = 1.4983. fineness (fine) a unit of proportion equal to 1/1000. This unit is used to express the purity of alloys of gold or other precious metals. A statement that a gold bar is "999 fine" means that the bar contains at most 0.1% other metals. See also per mill. finger [1] a traditional unit of distance equal to 2 nails or 4.5 inches (11.43 centimeters). This unit represents the length of the middle finger, from the tip to the joint where the finger is attached to the hand. finger [2] an informal name for the digit, a unit of distance equal to 3/4 inch. Finsen unit (FU) a unit formerly used in medicine to measure the intensity of the ultraviolet light used in various medical treatments. Ultraviolet radiation of the standard wavelength 296.7 nanometers has intensity 1 Finsen unit if its energy density is 105 watts per square meter. The unit honors the Danish physician Niels Finsen (1860-1904), who received the Nobel prize for medicine in 1903 for his research on the use of light in the treatment of various diseases. firkin (fir) [1] a traditional unit of volume equal to 1/4 barrel. Since barrels are of various sizes, the capacity of a firkin varies. Based on the standard U.S. barrel of 31.5 gallons, a firkin would equal 7.875 gallons, 1.05 cubic feet, or about 29.8 liters. Traditional British barrels and firkins are larger; in the imperial system a firkin holds 1.445 cubic feet or 40.91 liters. The unit is of Dutch origin, and its name is based on the Dutch word veer for four. firkin (fir) [2] a traditional unit of weight for butter and soap, equal to 4 stone or 56 pounds (about 25.40 kilograms). flagon a traditional unit of liquid volume, generally equal to the wine (or U.S. liquid) gallon (about 3.785 liters). A flagon is a large, narrow-necked pitcher or bottle. flask a commercial unit of weight formerly used to measure liquid mercury (or quicksilver, as it was called). The flask was equal to 76.5 pounds avoirdupois, which is equivalent to almost exactly 34.7 kilograms. flock an old English unit of quantity equal to 2 score or 40. floor another name for a story as a unit of height for buildings. In Britain, the floor above the ground floor is usually called the first floor, so a three-floor building has four stories. In the U.S., the numbering of floors is most often the same as the numbering of stories. flops a unit of computing power equal to one floating point operation per second. In computer science, there is a distinction between fixed point numbers (which have a fixed number of decimal places) and floating point numbers (which are stored with as many digits as the computer's design allows). A floating point operation is an addition or subtraction of two floating point numbers. As of the mid-1990's, the power of supercomputers was being measured in teraflops (Tflops): trillions of floating point operations per second. fluid dram or fluidram (fl dr) a unit of volume in the traditional apothecary system, equal to 1/8 fluid ounce. This unit is usually called the fluid dram or fluidram to avoid confusion with the weight dram. The U. S. fluid dram contains about 0.225 586 cubic inches or 3.696 691 milliliters. In the British imperial system, the fluid dram is about 0.216 734 cubic inches or 3.551 633 milliliters. fluid ounce (fl oz) a traditional unit of liquid volume, called the fluid ounce to avoid confusion with the weight ounce. In the U. S. customary system there are 16 fluid ounces in a pint, so each fluid ounce represents 1.804 687 cubic inches or 29.573 531 milliliters. In the British imperial system there are 20 fluid ounces in an imperial pint, so each fluid ounce represents about 1.733 871 cubic inches or 28.413 063 milliliters. A fluid ounce of water weighs just a bit more than one ounce avoirdupois. flux unit (fu) a former name for the jansky, a unit used in astronomy to measure the strength of a radio signal received from an object in the sky. fod the traditional Danish foot, equal to about 12.365 inches or 31.41 centimeters. foot (ft or ') a traditional unit of distance. Almost every culture has used the human foot as a unit of measurement. In England the Roman foot (29.6 centimeters) was replaced in the eighth century by the much larger Saxon foot (33.5 centimeters) and then in the thirteenth century by the modern foot (1/3 yard or about 30.5 centimeters). The length of the foot was standardized by standard yardsticks kept in England as early as the medieval period. Late in the nineteenth century, after both Britain and the U.S. signed the Treaty of the Meter, these standards were set aside in favor of the new metric standards. The foot was then defined to equal exactly 1200/3937 meter, or about 30.480 060 96 centimeters. This unit is called the survey foot in the United States. In 1959, the English-speaking countries agreed to redefine the foot to equal exactly 30.48 centimeters. This metric unit, called the international foot, is the current standard for traditional distance measurement in the English-speaking world. The foot is divided into 12 inches, 3 hands, 4 palms, or 16 digits. football field a common informal unit of distance in the United States. Americans aren't quite agreed as to whether the unit is exactly 100 yards (91.44 meters), the distance between the goal lines on an American football field, or 120 yards (109.728 meters), the distance including the two end zones. (Canadian football fields are 10 yards longer.) The football field is also used sometimes as an informal unit of area; including the end zones, an American football field represents an area of 1.3223 acre or about 0.535 hectare. foot candle (fc) a traditional unit of illumination, defined as the illumination received by a surface at a distance of one foot from a source of intensity one international candle. The "international candle" was the predecessor of the candela as the standard unit of light intensity. Illumination is now measured in lux; one foot candle equals 10.764 lux. foot of head (ft hd) a traditional unit of water pressure used in plumbing and hydraulics. "Head" is short for "headwaters"; it refers to the depth of the water upstream from the point at which the pressure is measured. One foot of head is equivalent to a pressure of 0.433 lb/in2, 2.989 kilopascals (kPa), 29.89 millibars (mb), or 0.882 inches of mercury (in Hg). foot per minute (ft/min or fpm) a traditional unit of velocity or flow rate. One foot per minute equals exactly 30.48 cm/min, 5.08 mm/s, or 0.018 288 miles per hour. foot per second (ft/s or ft/sec or fps) a traditional unit of velocity. One foot per second equals exactly 15/22 mile per hour, or exactly 1.097 28 kilometers per hour. foot pound (ft-lb) a traditional unit of work, equal to the work done by a force of one pound acting through a distance of one foot. This is equivalent to approximately 1.355 818 joule, 1.285 07 x 10-3 Btu, or 0.323 832 (small) calorie. foot poundal a unit of work, equal to the work done by a force of one poundal acting through a distance of one foot. One foot poundal is equivalent to about 0.031 081 foot pound, or 0.042 140 joule. force a measurement of wind velocity based on the Beaufort scale. fortnight a traditional English unit of time equal to 2 weeks or 14 days. The word is a contraction of "fourteen nights." fot the traditional Swedish foot, also called the Stockholm foot, equal to 11.689 inches or 29.69 centimeters. fother a traditional English unit of weight, used primarily for lead. The fother was always a little smaller than a ton; in one typical version still being used in the nineteenth century it was equal to 19.5 hundredweight or 2184 pounds. fourth a unit used in music to describe the ratio in frequency between notes. Two notes differ by a fourth if the higher note has frequency exactly 4/3 times the frequency of the lower one. On the standard 12-tone scale, the perfect fourth is approximated as 5 half steps, corresponding to a frequency ratio of 25/12 = 1.3348. fpm, fps common abbreviations for foot per minute (ft/min) and foot per second (ft/s or ft/sec), traditional units of flow (see above). f ratio (f/) a measure of the light-gathering power of the lenses in cameras and telescopes. The f ratio, expressed as "f/4" or whatever, is the focal length of the lens (the distance from the lens to the point where light is focused) divided by the aperture (the diameter of the lens, which in the case of cameras may be reduced by the shutter mechanism). Thus "f/4," for example, indicates that the focal length is 4 times the aperture. In cameras the f ratio is proportional to the square root of the exposure time, so in comparison to an an f/4 lens an f/8 lens requires a four times ((8/4)2) longer exposure than than the f/4 lens. The focal length of a camera's lens cannot be changed, so the shutter is used to reduce the aperture when longer exposure times are needed in order to match the available light to the speed of the film. Because of this connection with exposure times, the f ratio is often said to express the "speed" of a lens. franklin (Fr) a CGS unit of electric charge equal to 3.3356 x 10-10 coulombs or 1 statcoulomb. An electrostatic charge of one franklin exerts a force of one dyne on an equal charge at a distance of one centimeter. The unit honors Benjamin Franklin (1706-1790), one of the leaders of the American Revolution, who was also an early investigator of electricity. freight ton originally a unit of volume equal to 40 cubic feet; see ton [5]. More recently, another name for the revenue ton. French a unit of distance used for measuring the diameters of small tubes, fiber optic bundles, etc. One French is equal to 1/3 millimeter (about 13.123 mils). fuder a traditional German unit of liquid volume. A "fuder" is a cartload. In most of the German states the traditional fuder held about 9 hectoliters (roughly 238 U.S. gallons), making the fuder about the same size as the British tun or the French wine tonneau. In the Mosel wine region of Germany, a fuder is now a metric unit equal to 10 hectoliters (1 cubic meter, or 264.17 U.S. gallons). In Austria, the traditional fuder was equal to 18.11 hectoliters (478.42 U.S. gallons), twice the size of the German unit. In Belgium today, the fuder (or foudre) is a metric unit equal to 30 hectoliters (792.52 U.S. gallons). Fujita-Pearson scale (F) an empirical scale for estimating the wind speed of a tornado from the damage it causes. The scale, developed by the American meteorologists Theodore Fujita and Allen Pearson, ranges from F0 to F5. funt a traditional Russian unit of weight or mass, corresponding the German pfund (which is pronounced "funt"). The funt is equal to about 0.9028 pound avoirdupois or 409.5 grams. The plural is funte. furlong (fur) a traditional unit of distance. Long before the Norman Conquest in 1066, Saxon farmers in England were measuring distance in rods and furlongs and areas in acres. The word "furlong", from the Old English fuhrlang, means "the length of a furrow." A furlong equals 40 rods, which is exactly 220 yards, or 660 feet, or 1/8 mile. One furlong is exactly 201.168 meters, so a 200-meter dash covers a distance very close to a furlong. The length of horse races is often stated in furlongs. fuss the German foot (see foot, above). The length of the fuss varied somewhat; the Viennese version was equal to 12.444 inches or 31.608 centimeters, while the Rheinfuss (Rhine foot), used in much of western and northern Germany, was equal to 12.357 inches or 31.387 centimeters. The plural is fusse. G g a unit of acceleration equal to the average acceleration produced by gravity at the Earth's surface. (The actual acceleration of gravity varies from place to place, depending on latitude, altitude, and local geology). By agreement among physicists, 1 g is defined to be exactly 9.806 65 meters per second per second, or about 32.1740 feet per second per second. The unit was used first in aeronautical and space engineering, where it is important to limit the accelerations experienced by the crew members of aircraft and spaceships: the "g forces," as they are called. This use has become familiar through the space program, and now a variety of accelerations are measured in g's. The name grav is sometimes used for this unit. Note that g is also the symbol for the gram. G informal abbreviation in computer science for 230 = 1 073 741 824. See also giga- [2]. gage alternate spelling for gauge (see below). galactic year the unit of time in which the Solar System makes one revolution around the Milky Way galaxy. The galactic year is estimated to be about 225 million ordinary years. The age of the Solar System is about 20 galactic years. galileo (Gal or gal) the CGS unit of acceleration. One galileo is an acceleration of 1 centimeter per second per second. This unit is used by geologists, who make careful measurements of local variations in the acceleration of gravity in order to draw conclusions about the geologic structures underlying an area. These variations are typically measured in milligals (mGal). One Gal is approximately 0.001 019 7 g, so a milligal is a very small acceleration, about 10-6 g. The name of the unit honors the Italian astronomer and natural philosopher Galileo Galilei (1564-1642), who proved that all objects at the Earth's surface experience the same gravitational acceleration. To avoid confusion with the symbol for the gallon, and to conform to the usual metric style, the symbol for this unit should be Gal rather than gal. gallon (gal) [1] a traditional unit of liquid volume, derived from the Roman galeta, which originally meant a pailful. Gallons of various sizes have been used in Europe ever since Roman times. In the United States, the gallon is legally defined as exactly 231 cubic inches; this is equal to the old English wine gallon, standardized during the reign of Queen Anne in 1707. The U. S. gallon holds 4 liquid quarts or exactly 3.785 411 784 liters. American colonists were also familiar with an older unit, the Elizabethan beer and ale gallon, which held 282 cubic inches (4.621 liters). gallon (gal) [2] a traditional unit of dry volume. The U. S. dry gallon is 1/2 peck, or 4 dry quarts, or 268.8025 cubic inches, or approximately 4.404 884 liters. This unit is the English corn or grain gallon, standardized during the reign of Elizabeth I in the sixteenth century. The earliest official definition of a dry gallon in Britain is a 1303 proclamation of Edward I, where the gallon is defined as the volume of 8 pounds of wheat; the current U.S. gallon contains about 7.5 pounds of wheat. gallon (gal) [3] currently the British use a larger gallon than either of the American gallons. The Imperial Weights and Measures Act of 1824 established a new unit for all volumes, liquid or dry, replacing all the other gallons in previous use in Britain. The imperial gallon, designed to contain exactly 10 pounds of distilled water under precisely defined conditions, holds exactly 4.546 09 liters or approximately 277.4194 cubic inches. The imperial gallon equals 1.20095 U.S. liquid gallons or 1.03206 U.S. dry gallon. gamma [1] a unit of magnetic flux density equal to 10-9 tesla (1 nanotesla) or 10-5 oersted (10 ¥ìOe). In geophysics, small changes in the Earth's magnetic field are traditionally stated in gammas. The nanotesla (nT) is now recommended for these measurements. gamma [2] an informal metric unit of mass equal to 1 microgram (¥ìg). garnets a traditional unit of liquid volume in Russia. The garnets is equivalent to approximately 3.28 liters; this is about 3.47 U.S. quarts or 2.89 British imperial quarts. gauge (ga) [1] a traditional unit measuring the interior diameter of a shotgun barrel. The gauge of a shotgun was the number of lead balls, each of a size just fitting inside the barrel, which were required to make up a pound. In other words, if a lead ball weighing 1/12 pound just fit in the barrel of a shotgun, then it was a 12-gauge shotgun. Today, the internal diameters for each gauge number are read from a table. gauge (ga) [2] a unit expressing the fineness of a knitted fabric, equal to the number of loops per 1.5 inches (38.1 millimeters). The same unit is also used to express the size of the knitting needles used to create a fabric of that fineness. gauge (ga) [3] a traditional unit measuring the diameter (or the cross-sectional area) of a wire. Various wire gauge scales have been used in the U.S. and Britain. In traditional scales, larger gauge numbers represent thinner wires. In the American Wire Gauge (AWG) scale, 0000 gauge represents a wire having a diameter of 0.46 inch and 36 gauge represents a diameter of 0.005 inch (5 mils). Diameters for the other gauges are obtained by geometric interpolation, meaning that the ratio between successive diameters is a constant, except for necessary roundoff. Thus n gauge wire has a diameter of .005¡¤92((36-n)/39) inch (gauges 00, 000, and 0000 are interpreted as -1, -2, and -3, respectively, in this calculation). The metric wire gauge number is equal to 10 times the diameter of the wire, in millimeters; thus a metric 8 gauge wire has diameter 0.8 millimeters. Links: table of AWG, British Standard, and metric wire gauges, from the Alpha Wire Company (click on AWG/Imperial/Metric Comparison: Solid Conductor); cloth weaving wire gauge diameters from Screen Technology Group Inc. gauge (ga) [4] a traditional unit measuring the thickness of sheet metal. Larger gauge numbers represent thinner metal: 10 gauge represents a thickness of 0.1345 inch (3.416 millimeters) and each increase of 1 in the gauge number corresponds to a reduction of about 10% in the thickness. A table is provided. gauss (G or Gs) the CGS unit of magnetic flux density. A field of one gauss exerts, on a current-carrying conductor placed in the field, a force of 0.1 dyne per ampere of current per centimeter of conductor. One gauss represents a magnetic flux of one maxwell per square centimeter of cross-section perpendicular to the field. In SI units, one gauss equals 10-4 tesla. The unit is named for the German mathematician and astronomer Karl Friedrich Gauss (1777-1855). gee pound another name for a slug. generation (gen) an informal unit of time. Roughly speaking, a generation is the average length of time between the birth of a parent and the birth of the child. This leaves a question, however: should just the father, or just the mother, or both parents be included in the calculation? Various answers to this question, plus a lack of consistent data, have led to a range of estimates for the length of a generation, from about 25 to 35 years. Genealogists tend to use the higher figures, anthropologists the lower ones. There is some research suggesting that the approximate length of the generation in the world today is about 28 years. geographical mile another name for the nautical mile, especially the Admiralty mile (6080 feet or 1853.184 meters). For a (different) German use of the term geographische meile, see meile. g/g a symbol for "gram per gram", a unit of mass concentration. For example, a concentration of 0.02 g/g, or 2% g/g, means that the substance being measured comprises 2% of the mass of the mixture in which it is found. This symbol is equivalent to the traditional symbol w/w. gibi- (Gi-) a binary prefix meaning 230 = 1 073 741 824. This prefix, adopted by the International Electrotechnical Commission in 1998, replaces giga- for binary applications in computer science. The prefix is a contraction of "gigabinary." gig an informal contraction of the gigabyte (GB) common in computer science, especially as a unit of storage capacity. giga- (G-) a metric prefix denoting 109 or one billion (in the American meaning of the word billion). This prefix was coined from the Greek word gigas, meaning giant. The Greeks pronounced the word with a hard g, as in "gig." However, the prefix is usually pronounced with a soft g, as in "jig." Dictionaries usually list the hard g pronunciation as a second choice. gilbert (Gi) the CGS unit of magnetomotive force, equal to 10/4pi = 0.795 775 ampere-turns. The unit is named for the English physicist and physician William Gilbert (1540-1603). gill (gi) a traditional unit of volume for liquids, especially wine and other alcoholic beverages. The gill is 1/4 pint. In the U. S. customary system, one gill is equal to 1/2 cup, 4 fluid ounces, 7.21875 cubic inches, or about 118.3 milliliters. In the British Imperial system, the gill equals 5 fluid ounces, 8.670 cubic inches, or about 137.7 milliliters. gillion an informal alternate name for the number 109, called "billion" in America but often called "milliard" in France and "thousand million" in Britain. The reasoning here is that if mega- means a million and tera- a trillion, then giga- should mean a gillion! gin one of several spellings used in English for the jin, the Chinese weight unit also called the catty. glass [1] another name for the U.S. cup (236.6 milliliters). Doctors in the U.S. are fond of saying that everyone should drink 8 glasses of water a day, and this is the amount they have in mind for a glass. glass [2] an informal unit of volume used in Australian pubs. In the eastern states of Australia a glass of beer is usually 200 milliliters, but in Western Australia it is 285 milliliters. gm an incorrect symbol for the gram (see below). The correct symbol is g. GMT abbreviation for Greenwich Mean Time, the standard time of longitude 0¡Æ. This meridian of longitude, called the prime meridian, was fixed as the longitude of the Royal Greenwich Observatory in London by the International Meridian Conference of 1884. GMT is five hours later than U.S. Eastern Standard Time. The abbreviation UT (Universal Time) has largely replaced GMT. go a traditional Japanese unit of liquid volume. One go is about 180.39 milliliters, 0.3812 U.S. pint, or 0.3174 British imperial pint. goad a traditional unit of distance sometimes used in measuring cloth. One goad is equal to 54 inches or 1.5 yards (1.3716 meters). A goad was originally a spear; later it was a pointed rod used for prodding animals to get a move on. The unit must have originated as the length of such an instrument. gon another name for the grad (see below). googol a unit of quantity equal to 10100 (1 followed by 100 zeroes). The googol was invented by the American mathematician Edward Kasner (1878-1955) in 1938. According to the story, Kasner asked his nephew Milton Sirotta, who was then 8 years old, what name he would give to a really large number, and "googol" was Milton's response. Kasner also defined the googolplex, equal to 10googol, that is, 1 followed by a googol of zeroes. These inventions caught the public's fancy and are often mentioned in discussions of very large numbers. gpg customary symbol for grains per gallon (gr/gal), a traditional unit measuring the hardness of water. Water is "hard" if it contains dissolved minerals such as calcium or magnesium salts. 1 gpg is equivalent to about 17.118 milligrams per liter (mg/L). gpm, gps customary symbols for gallons per minute (gal/min) and gallons per second (gal/s), traditional units for measuring the flow of liquids. 1 gpm equals about 3.785 41 liters per minute (L/min) if U.S. gallons are meant, or exactly 4.546 09 L/min if British imperial gallons are meant. grad or grade (g or grd) [1] a unit of angle measurement, equal to 1/400 circle, 0.01 right angle, 0.9¡Æ, or 54'. This unit was introduced in France, where it is called the grade, in the early years of the metric system. The grad is the English version, apparently introduced by engineers around 1900. Although many calculators will display angle measurements in grads as well as degrees or radians, it is difficult to find actual applications of the grad today. grade [2] a measure of the steepness of a slope, such as the slope of a road or a ramp. Usually stated as a percentage, the grade is the same quantity known as the slope in mathematics: the amount of (vertical) change in elevation per unit distance horizontally ("rise over run"). Thus a 5% grade has an elevation gain of 0.05 meter for each meter of horizontal distance, or 0.05 foot for each foot of horizontal distance. The angle of inclination, in grads or grades [1], is not equal to the percentage grade in this sense: for a 5% grade the angle of inclination is about 2.86¡Æ or 3.18 grads. grade [3] a measure of quality for ball bearings. A bearing of grade g is required to be spherical to an accuracy of g parts per million (g 10-6). Thus lower grade numbers represent better bearings; a 25 grade bearing is spherical to within 25 ppm, but a 1000 grade bearing is only spherical to within 1000 ppm, or 0.1%. grain (gr) [1] a traditional unit of weight. The grain, equal to 1/480 troy ounce (see also pound [2]), or exactly 64.798 91 milligrams, was the legal foundation of traditional English weight systems, with various pounds being defined as a specified number of grains: 5760 grains in a troy pound and 7000 grains in an avoirdupois pound, for example. In the version of the troy system used by jewelers, there are 24 grains in a pennyweight and 20 pennyweight in an ounce. In the version used by apothecaries, there are 20 grains in a scruple, 3 scruples in a dram, and 8 drams in an ounce. grain (gr) [2] a unit of weight formerly used by jewelers in measuring diamonds and other precious stones. The jeweler's grain is exactly 1/4 carat. Now that the carat has been standardized at 200 milligrams, the jeweler's grain is exactly 50 milligrams, or approximately 0.7716 troy grain. grain (gr) [3] a traditional French unit of weight equal to 53.115 milligrams. gram (g) the basic unit of mass in the metric system. The name comes the Latin gramma, which was a small weight similar to the English grain. The gram was originally defined to be the mass of one cubic centimeter of pure water, but to provide precise standards it was necessary to construct physical objects of specified mass. One gram is now defined to be 1/1000 of the mass of the standard kilogram, a platinum-iridium bar carefully guarded by the International Bureau of Weights and Measures in Paris for more than a century. The gram is a small mass, equal to about 15.432 grains or 0.035 273 966 ounce. The original French spelling gramme is sometimes used. Note: The only correct symbol for the gram is g. The abbreviations "gm" and (worse) "gr" should never be used. gram atom, gram atomic weight older names for the atomic mass unit. gram calorie the CGS unit of heat energy; see calorie. gram mole, gram molecule, gram molecular weight (gmol or gmole) older names for the mole. gram rad a name sometimes used for the rad, a unit of radiation dose. gram weight an informal unit of weight equal to the force exerted on a mass of one gram by gravity at the Earth's surface: approximately 980.665 dynes, or about 0.0022045 pounds of force in the English system. gran a traditional German weight unit, varying in size but typically about 60 milligrams. grano a traditional Italian weight unit, varying in size but typically about 50 milligrams. The plural is grani. grav (g) another name for the unit of acceleration usually called the g (see above). gray (Gy) the SI unit of radiation dose. Radiation carries energy, and when it is absorbed by matter the matter receives this energy. The dose is the amount of energy deposited per unit of mass. One gray is defined to be the dose of one joule of energy absorbed per kilogram of matter, or 100 rad. The unit is named for the British physician L. H. Gray (1905-1965), an authority on the use of radiation in the treatment of cancer. great gross a traditional unit of quantity equal to a dozen gross, or 1728. great hundred a traditional unit of quantity equal to 120 instead of 100. The great hundred equals 6 score or 2 shocks. Merchants in medieval England used both the great hundred and the gross in specifying quantities, but more recently the gross has been much more common. Gregorian year a unit of time equal to exactly 365.2425 days, the average length of a year in the Gregorian calendar currently used throughout the world. See year [2]. grit a measure of fineness for abrasive materials such as sandpaper, sanding belts, or the finer materials used to polish optical surfaces. Originally the grit number was the number of holes in a standard screen; if the screen had, say, 240 holes, then the particles that would pass through the screen were described as 240 grit. However, very fine abrasive particles (such as 500 to 1000 grit) are too small to be screened in this way, so the measure is defined by tables giving the average particle size in micrometers for each grit size. A table of grit sizes is provided. gros a traditional French weight unit equal to 3 deniers (about 3.824 grams). gross (gro or gr) a unit of quantity equal to a dozen dozen, or 144. This commercial unit has been in use since at least the 1400's. gross ton the name "gross ton" is used in at least two ways: (1) as another name for the long ton of 2240 pounds (see ton [1]), and (2) as another name for the register ton, a unit of volume equal to 100 cubic feet (see ton [3]). gsm a common but non-standard symbol for grams per square meter (g/m2), the metric unit of density for paper. Paper density measured in this unit is often called grammage. gtt traditional pharmicist's abbreviation for a drop. Gunter's chain the traditional surveyor's chain equal to 4 rods; see chain. H h Planck's constant, equal to approximately 0.662 606 876 x 10-33 joule second, a fundamental constant of physics also used as a unit of "action" or of angular momentum in particle physics. The unit was defined by the German physicist Max Planck (1858-1947), who showed in 1900 that at atomic and subatomic scales energy occurs in discrete packets called quanta. Each quantum has energy hf, where f is the frequency of the radiation in hertz (see below). halakim (hl) plural of helek (see below). half a unit of proportion equal to 1/2. The English word "half," like the German prefix "halb-," is often placed before the name of a unit to create a combination which functions as a new unit equal to half the old one. Half dozen, half hour, and half gallon are typical and common examples. half life a unit of relative time measuring the rate at which a radioactive substance decays, or (more generally) the rate of decrease for any process that decreases exponentially. In the case of radioactivity, the half life is the time required for the activity to be reduced by half. After a second half life, the activity is again reduced by half, so it is then 1/4 the original activity. To reduce the activity to 0.1% of the original amount requires about 9.966 half lives. half step a unit used in music, equal to 1/12 octave. The half step measures the difference between adjacent notes in the standard 12-tone scale, as on a piano keyboard. Two notes differ in frequency by a half step if the higher one has frequency equal to 21/12 = 1.0595 times the frequency of the lower one. hand a traditional unit of distance, often used to measure the height of horses. One hand equals 4 inches, 1/3 foot, or 10.16 centimeters. hank a traditional measure of length for yarn. The length of yarn in a hank varies with the market and the material; for example, a hank of cotton yarn traditionally included 840 yards (768 meters) of yarn, while a hank of wool yarn was 560 yards (512 meters). In the U.S., however, a hank of wollen yarn is generally 1600 yards (1463 meters). For both cotton and wool, the hank is equal to 7 leas or to 12 cuts. In retail trade, a hank is often equal to 6 or 7 skeins of varying size. hardness an estimate of the hardness of a mineral made using the Mohs hardness scale. hartley (Hart) a unit of information content used in information and communications theory. The hartley is similar to the shannon. If the probability of receiving a particular message is p, then the information content of the message is -log10 p hartleys. For example, if a message is a string of 5 letters or numerals, with all combinations being equally likely, then a particular message has probability 1/365 and the information content of a message is 5(log10 36) = 7.7815 hartleys. One hartley equals log2 10 = 3.321 928 shannons. hartree (Eh) a unit of energy used in particle physics, equal to about 4.3598 x 10-18 joule or 27.212 electron volts. hat trick an informal unit of quantity, equal to 3. The unit is used in sports in counting goals, especially goals scored by the same player. It originated in cricket but is more familiar to North Americans in ice hockey. No one seems to know for sure why 3 goals came to be called a hat trick. According to one popular account, cricket clubs in England used to award a hat to a bowler who took 3 wickets with successive ballsbut some suspect this was an effect, rather than a cause, of the expression. head (hd) an informal unit of length, equal to the approximate length of a horse's head, used in expressing the results of a horse race. heaped bushel a traditional unit of volume in the United States, the heaped bushel is just what its name implies: the volume of a bushel container filled to overflowing. Officially, the heaped bushel was supposed to equal 1.278 regular, or "struck" bushels; this is a volume of 2748.237 cubic inches, 1.5904 cubic feet, or 45.036 liters. In practice, the heaped bushel was frequently interpreted as 1.25 bushels, which is equal to exactly 10 dry gallons, 2688.025 cubic inches, 1.5556 cubic feet, or 44.049 liters. heat index (HI or HX) a measure of the combined effect of heat and humidity on the human body. U.S. meteorologists compute the index from the temperature T (in ¡ÆF) and the relative humidity H (as a fraction; that is, H = 0.65 if the relative humidity is 65%). The formula used is HI = -42.379 + 2.04901523 T + 1014.333127 H - 22.475541 TH - .00683783 T2 - 548.1717 H2 + 0.122874 T2H + 8.5282 TH2 - 0.0199 T2H2. heat unit an alternate name, coined by Americans, for the British thermal unit (Btu). This is not the same as the Celsius heat unit (Chu). hebdo- an obsolete metric prefix denoting ten million (107). It was coined from the Greek hebdomos, seventh. hect- or hecto- (h-) a metric prefix denoting 100, coined from the Greek word hekaton for one hundred. hectare (ha) a metric unit of area, equal to 100 ares. One hectare is the area of a square 100 meters on each side: approximately 107 639.1 square feet or 2.471 054 acres. The hectare is now used in most countries to express those land areas traditionally measured by the acre in English-speaking countries. hectogram (hg) a common metric unit of mass, equal to 100 grams or about 3.5274 ounces. hectoliter (hL or hl) a common metric unit of volume. The hectoliter equals 100 liters, 0.1 cubic meter, 26.417 U.S. liquid gallons, 21.999 British imperial gallons, or 3.5315 cubic feet. hectopascal (hPa) an SI unit of pressure equal to 100 pascals. The hectopascal is identical to the millibar (mb). Following a 1986 recommendation of the World Meteorological Organization, meteorologists in most countries have switched from the millibar to the hectopascal for measurements of air pressure. helek (hl) a traditional Hebrew unit of time equal to 1/18 minute or 10/3 seconds. The plural is halakim. Halakim are used in formulas establishing the molad, the instant of new moon, which marks the start of the month in the Jewish calendar. hemisphere a traditional unit of solid angle equal to 1/2 sphere, 2pi steradians, or about 20 626.48 square degrees. henry (H) the SI unit of electric inductance. A changing magnetic field induces an electric current in a loop of wire (or in a coil of many loops) located in the field. Although the induced voltage depends only on the rate at which the magnetic flux changes, measured in webers per second, the amount of the current depends also on the physical properties of the coil. A coil with an inductance of one henry requires a flux of one weber for each ampere of induced current. If, on the other hand, it is the current which changes, then the induced field will generate a potential difference within the coil: if the inductance is one henry a current change of one ampere per second generates a potential difference of one volt. The henry is a large unit; inductances in practical circuits are measured in millihenrys (mH) or microhenrys (¥ìH). The unit is named for the American physicist Joseph Henry (1797-1878), one of several scientists who discovered independently how magnetic fields can be used to generate alternating currents. The plural is sometimes spelled henrys, but in English it is correct to spell it henries. heptad a unit of quantity equal to 7. hertz (Hz) the SI unit of frequency, equal to one cycle per second. The hertz is used to measure the rates of events that happen periodically in a fixed and definite cycle; the becquerel, also equal to one "event" per second, is used to measure the rates of things which happen randomly or unpredictably. Multiples of the hertz are common: the frequencies of radio and television waves are measured in kilohertz (kHz), megahertz (MHz), or even gigahertz (GHz), and the frequencies of light waves in terahertz (THz). The unit is named for the German physicist Heinrich Rudolf Hertz (1857-1894), who proved in 1887 that energy is transmitted through a vacuum by electromagnetic waves. hexad a unit of quantity equal to 6. hexit a unit of information equal to 4 bits or 1/2 byte. A string of 4 bits has 16 possible states (0-15) and is usually represented as a single base-16, or hexadecimal, digit (in the hexadecimal system the letters A through F are used to represent the numbers 10 through 15, respectively). A hexit of data is also known as a nibble or a quadbit. hide an old English unit of land area. The hide was the amount of land that could be cultivated by a single plowman and thus the amount of land necessary to support a family. Depending on local conditions, this could be as little as 60 acres or as much as 180 acres. 120 acres is a commonly used estimate. hogshead (hhd) a traditional unit of volume for liquids. Originally the hogshead varied with the contents: there were different sizes for wine, ale, and beer. In the United States, a hogshead is defined to hold 2 barrels, or 63 gallons; this was the traditional British wine hogshead. It is equal to exactly 14 553 cubic inches, or about 8.422 cubic feet (238.48 liters). In the British imperial system, the hogshead equals 1/2 butt, or 52.5 imperial gallons (8.429 cubic feet, or 238.67 liters). Thus the British and American hogsheads are almost exactly the same size. No one seems to know for sure how this unit got its unusual name. homestead a historic unit of area in the United States, equal to 160 acres (64.75 hectares). Under the Homestead Act, passed by Congress in 1862, settlers in the western states were allowed to take title to a homestead of 160 acres of land by registering a claim, settling on the land, and cultivating it. A homestead equals 1/4 square mile, or 1/4 section in U.S. government terminology. horsepower (hp) a unit of power representing the power exerted by a horse in pulling. The horsepower was defined by James Watt (1736-1819), the inventor of the steam engine, who determined after careful measurements that a horse is typically capable of a power rate of 550 foot-pounds per second. This means that a horse, harnessed to an appropriate machine, can lift 550 pounds at the rate of 1 foot per second. Today the SI unit of power is named for Watt, and one horsepower is equal to approximately 745.6999 watts. (Slightly different values have been used in certain industries.) horsepower hour (hp hr) a unit of work or energy equal to the work done at the rate of 1 horsepower for 1 hour. The horsepower hour equals 1 980 000 foot pounds or approximately 2.685 megajoules, 2545 Btu, 641.1 (large) Calories, or 745.7 watt hours. hour (h or hr) [1] a traditional unit of time, equal to 60 minutes, or 3600 seconds, or 1/24 day. The custom of dividing the daylight into 12 hours goes back at least as far as the Babylonians, who liked to divide units by 12 because groups of 12 are easily divided into halves, thirds, or fourths. Later, when people wanted to express times at night, it was natural to divide the night into 12 hours as well, making 24 hours in the day. The word comes from an ancient Greek word hora which originally meant a season, especially a religious season, and hence a "defined" period of time. hour (h or hr) [2] a unit of angular measure used in astronomy, equal to 1/24 circle or 15¡Æ. Objects are located in the sky by latitude and "right ascension". Latitude is measured in degrees from the equator to the poles, just as it is on the surface of the earth. Longitude is replaced by right ascension, measured in hours from the longitude, traditionally known as the First Point of Aries, at which the Sun crosses the equator on its northward journey in the spring. house in astrology, each sign of the Zodiac is called a house; thus the house can be considered a unit of angle measure equal to 1/12 circle or 30¡Æ. hu a traditional Chinese unit of liquid volume. The hu contains about 51.77 liters, 13.676 U.S. gallons, or 11.389 British imperial gallons. hubble a unit of distance sometimes used in astronomy. The hubble is a gigantic unit, equal to 109 light years. This is 9.4605 x 1021 kilometers (9.4605 yottameters, if you please), 5.8785 sextillion miles (U.S.), 63.240 x 1012 astronomical units, or 306.59 megaparsecs. In practice, most astronomers use the megaparsec for measuring such stupendous distances. The unit honors the American astronomer Edwin Hubble (1889-1953), who discovered the uniform expansion of the Universe explained by the Big Bang theory. hundred an old English unit of area equal to 100 hides (see above). This is roughly 12 000 acres, 5000 hectares, or 18.75 square miles. The hundred is approximately the area of a village with its associated fields, so the name "hundred" came to mean a minor division of a shire or county. This use carried over to the American colonies, where, for example, many of the early settlements in Virginia were called hundreds. hundredweight (Cwt) a traditional unit of weight equal to 1/20 ton. The hundredweight is the English version of a commercial unit used throughout Europe and known in other countries as the quintal or the zentner. In general, this unit is larger than 100 pounds avoirdupois, so to fit the European market the hundredweight was defined in England as 112 pounds avoirdupois (about 50.8023 kilograms) rather than 100 pounds. This definition apparently dates from about the middle of the 1300's. In the United States, where there wasn't so much need to align the unit with the quintal and zentner, the hundredweight came to equal exactly 100 pounds (about 45.3592 kilograms). To distinguish these two units, the British version is often called the long hundredweight and the American is called the short hundredweight or cental. The C in the symbol is of course the Roman numeral 100. I i a mathematical unit equal to the square root of -1. Although often called the imaginary unit, i is quite real in many applications. For example, in vector geometry it is used to represent a counterclockwise rotation by 90¡Æ. The Swiss mathematician Leonhard Euler (1707-1783) introduced the symbol i for the imaginary unit in 1777. immi a traditional Swiss unit of liquid volume. The immi has been brought into the metric system and now equals exactly 1.5 liters (about 3.17 U.S. liquid quarts). imperial a large wine bottle holding about 6 liters, 8 times the volume of a regular bottle. Also called a methuselah. imperial units the units of the British imperial system, adopted by Parliament in 1828. The basic units of the system are the foot, the avoirdupois pound, and the imperial pint. inch (in or ") [1] a traditional unit of distance, equal to 1/12 foot or 1/36 yard. One inch is now defined to be exactly 2.54 centimeters. The Old English word ynce is derived from the Latin uncia, meaning a 1/12 part; thus "inch" and "ounce" actually have the same root. However, in many other European languages the word for inch also means thumb: this true of the Dutch duim, Swedish tum, French pouce, and Spanish pulgada, for example. Thus the inch is often thought of as the width of a thumb. inch of mercury (in Hg) a traditional unit of atmospheric pressure. In the United States, atmospheric pressure is customarily expressed as the height of a column of mercury exerting the same pressure as the atmosphere. When a traditional mercury barometer is used, this height is read directly as the height of the mercury column. In the symbol for the unit, Hg is the chemical symbol for mercury. The pressure of one inch of mercury is about 0.491 pounds per square inch, or 33.87 millibars, or 3.387 kilopascals. inch of water column (in WC) a traditional unit of pressure, used in plumbing to describe both water and gas pressures. A pressure of 1 inch of water equals 249.09 pascals, which is 2.4909 millibars, about 0.0360 pounds per square inch (psi) or about 0.07348 inches of mercury. inch of water gauge (in wg or "wg) another common name for the inch of water column. The word "gauge" (or "gage") after a pressure reading indicates that the pressure stated is actually the difference between the absolute, or total, pressure and the air pressure at the time of the reading. inch pound (in lb) a traditional unit of work or energy, equal to 1/12 foot pound, about 0.112 985 joule or 1.0709 x 10-4 btu. inhour (inhr) a unit used in nuclear engineering to describe the "reactivity" of a nuclear reactor. Reactivity is defined to be 1 divided by the time the reactor has been operating (in hours), so 1 inhr = 1 hr-1. international foot the current foot unit of the English-speaking countries, equal to exactly 30.48 centimeters. See foot for additional information. international nautical mile the nautical mile as currently defined by international agreement, equal to exactly 1852 meters or 6076.11549 feet. This long name is sometimes used to distinguish the current nautical mile from older units. international unit (IU) a unit used to measure the mass of certain vitamins and drugs based on their expected effects. For each substance to which this unit applies, there is an international agreement specifying the biological effect expected with a dose of 1 IU. Other quantities of the substance are then expressed as multiples of this standard. Examples: 1 IU represents 45.5 micrograms of a standard preparation of insulin or 0.6 microgram of a standard preparation of penicillin. Consumers most often see IU's on the labels of vitamin packages: the equivalent of 1 IU is 0.3 microgram (0.0003 mg) for vitamin A, 50 micrograms (0.05 mg) for vitamin C, 25 nanograms (0.000 025 mg) for vitamin D, and 2/3 milligram for vitamin E. ipy an abbreviation for inches per year (in/yr), a traditional unit for corrosion rates. Irish mile the traditional mile in Ireland is 6720 feet, which is 1.272 727 English mile or 2.048 256 kilometers. The discrepancy arose because the Irish perch, or rod, was standardized at 21 feet instead of the English figure of 16.5 feet. Just as in England, the Irish chain was equal to 4 perches (84 feet instead of 66 feet), the Irish furlong was equal to 10 chains (840 feet instead of 660 feet) and the mile was equal to 8 furlongs. ISO the initials of the International Standards Organization. Based in Switzerland, the ISO sets international industrial standards, including standards for the use of units of measurements. For ISO film ratings, see ASA. J jansky (Jy) a unit used in radio astronomy to measure the strength, or more precisely the flux density, of radio signals from space. In measuring signal strength, it's necessary to take into account both the area of the receiving antenna and the width of frequency band in which the signal occurs. Accordingly, one jansky equals a flux of 10-26 watts per square meter of receiving area per hertz of frequency band (W/m2Hz). Although it is not an SI unit, the jansky is approved by the International Astronomical Union and is widely used by astronomers. It honors Karl G. Jansky (1905-1950), the American electrical engineer who discovered radio waves from space in 1930. The jansky is sometimes called the flux unit. jar a traditional unit of electric capacitance, approximately equal to the capacitance of one of the Leiden jars used in electrical experiments as long ago as the eighteenth century. Benjamin Franklin is said to have measured the storage power of his electrical equipment in jars. There are 9 x 108 jars in a farad, so 1 jar is approximately 1.1 microfarad. jerk a unit of change in acceleration sometimes used by engineers. This is not as silly as it sounds, because when we're in a vehicle and feel a jerk, we are in fact experiencing a change in the acceleration of the vehicle. One jerk is equal to a change in acceleration of one foot per second per second in one second, that is, 1 ft/sec3. One jerk equals about 0.03108 g/sec or 0.3048 m/s3. jeroboam a large wine bottle holding about 3 liters, 4 times the volume of a regular bottle. jiffy a unit of time used in computer engineering. A jiffy is the length of one cycle, or tick, of the computer's system clock. In the past, this was often equal to one period of the alternating current powering the computer: 1/60 second in the U.S. and Canada, usually 1/50 second elsewhere. More recently the jiffy has become standardized, more or less, as 0.01 second (10 milliseconds). The word jiffy, with its ordinary meaning of an instant or very brief time, appeared in English during the eighteenth century, but its origin is not known. jigger a unit of volume for liquor, usually considered equal to 1.5 (U.S.) fluid ounce or 44.360 milliliters. jin a traditional unit of weight in China, comparable to the English pound. During the European colonial era the jin was identified with the catty, a Malay unit widely used in various forms throughout East and Southeast Asia. Like the catty, the jin was then equal to 1 1/3 pounds or 604.79 grams. Traditionally, it was divided into 16 liang. In modern China, however, the jin has been identified as a metric unit equal to exactly 500 grams (1.1023 pounds), and it is divided into 10 liang. The spellings chin and gin also have been used for this unit. jo an informal unit of area used in Japan to measure the size of rooms in houses and apartments. One jo is the area of a traditional tatami mat, 180 by 90 centimeters or 1.62 square meters (1.94 square yards). joch a traditional unit of area in German speaking countries, especially in Austria. One joch is the area of a square 40 klafters (about 83 yards) on a side. This comes to 0.5755 hectare or about 1.422 acres. The plural is joche. Joch is also the word for a yoke in German, so presumably this unit represents an area that could be plowed in a day by a yoke of oxen. joule (J) the SI unit of work or energy, defined to be the work done by a force of one newton acting through a distance of one meter. Equivalently, one joule is the kinetic energy of a mass of one kilogram moving at a velocity of 1 m/sec. This is the same as 107 ergs in the CGS system, or approximately 0.737 562 foot-pound in the traditional English system. In other energy units, one joule equals about 9.478 170 x 10-4 Btu, 0.238 846 (small) calories, or 2.777 778 x 10-4 watt hour. The joule is named for the British physicist James Prescott Joule (1818-1889), who demonstrated the equivalence of mechanical and thermal energy in a famous experiment in 1843. Although Joule pronounced his name "jowl", the unit is usually pronounced "jew'l". journal a traditional unit of land area in France, equal to the area that could be plowed in a day (jour is the French word for day). The unit varied from one region to another, generally in the range 0.3-0.45 hectare (0.75-1.1 acres). JTU abbreviation for Jackson turbidity unit, a unit formerly used in measuring water quality. The turbidity of water was measured by lighting a candle under a tall glass tube and filling the tube with the water sample until a observer looking down the tube could no longer see the candle flame through the water. The height of the water column determines the turbidity in Jackson turbidity units, using a table constructed for this purpose. Turbidity is usually measured now in a different unit, the NTU, using an instrument called a nephelometer. jug an informal name for the Scots pint, a unit of volume equal to about 1.791 U.S. liquid quarts or 1.695 liters. Julian day (JD) a continuous count of days beginning with January 1, 4713 BC (-4712 CE). Julian days are named for the French scholar Julius Scaliger (1540-1609), who introduced the idea in 1582 (the same year the Gregorian calendar was proclaimed). They are often used by astronomers and sometimes used by historians to provide a precise date for an event, independent of all calendar systems. Scaliger picked 4713 BC for the start of the count because this was earlier than all known historical records and happened to be a convenient starting point for several chronological and astronomical cycles. The Julian day is taken to begin at noon Universal Time. The first moment of the year 2000 CE, at the International Date Line, was also the start of JD 2 451 544. Julian epoch (J) a measure of time used in astronomy. The word epoch comes from Greek, and means a fixed or standard instant of time. Other times are stated with reference to this fixed time using years and fractions of years. In 1984, astronomers agreed to fix the standard epoch at 12 hours Universal Time of 2000 January 1 (JD 2 451 545.0). This instant is designated J2000.0. Other times are specified with reference to this time using a year of length 365.25 days, the average length of the year in the Julian calendar. This means that J2001.0, for example, is 18 hours Universal Time of 2001 January 1 (exactly 365.25 days after J2000.0). Julian year the average length of the year in the Julian calendar, equal to exactly 365.25 days. See year [2]. Jupiter a unit of mass, now being used in astronomy to express the masses of new planets being discovered in orbit around various stars. It's equal to the mass of the planet Jupiter, estimated to be about 1.899 x 1024 metric tons, or, if you please, 1.899 yottatonnes (Yt). K K informal abbreviation for thousand, used in expressions where the base unit is understood, such as "10K run" (10 kilometers) or "700K disk" (700 kilobytes). Note that "K" is also the symbol for the kelvin (see below). Also note that the symbol for the metric prefix kilo- (1000) is actually k-, not K-. karat (kt) a traditional measure of proportion, equal to 1/24, used by jewelers to express the purity of gold alloys. Thus "14-karat gold" is legally required to be at least 14/24, or 58.3%, gold. In Britain the unit is spelled carat, just like the weight unit for diamonds and other precious stones. American jewelers spell the unit of gold purity with the "k" and the weight unit with the "c" in order to distinguish more clearly between them. katal (kat) a unit of catalytic activity, used especially in the chemistry of enzymes. A catalyst is a substance which initiates or facilitates a chemical reaction. Enzymes are proteins which act as catalysts within the bodies of living plants and animals. A catalyst has an activity of one katal if it enables a reaction to proceed at the rate of one mole per second. The unit is pronounced "cattle." The katal was officially added to the International System at the 21st General Conference of Weights and Measures in October 1999. kati a traditional Malaysian unit of weight, usually spelled catty in English. kayser (K) a CGS unit used to measure light and other electromagnetic waves. The "wave number" in kaysers equals the number of wavelengths per centimeter. The unit honors J. H. G. Kayser (1853-1940), who compiled a giant atlas of chemical spectra. The unit is often abbreviated K, although this conflicts with the abbreviation for the kelvin. keddah a traditional Egyptian unit of liquid volume, also used in other parts of the Middle East. The keddah is equal to about 2.0625 liter (about 2.18 U.S. liquid quarts or 1.815 British imperial quarts). keel a British unit of weight used for measuring coal. After considerable variation, the keel of coal was standardized in 1695 as 21.2 long tons, or 47 488 pounds. This is the weight of coal which used to be carried by barges on the river Tyne in northern England; the barges were also called keels, from the Dutch word kiel for such a ship. keg [1] a traditional unit of volume or quantity, varying with the item contained in the keg. A keg of herring, for example, contains 60 fish. A keg of wine is frequently 12 U.S. gallons (about 45.42 liters). keg [2] a traditional unit of weight used to measure nails. A keg of nails weighs 100 pounds and thus has a mass of about 45.359 kilograms. kelvin (K) the fundamental SI unit of temperature, previously called the degree Kelvin (¡ÆK). One kelvin represents the same temperature difference as one degree Celsius. In 1967 the General Conference on Weights and Measures defined the temperature of the triple point of water (the temperature at which water exists simultaneously in the gaseous, liquid, and solid states) to be exactly 273.16 kelvins. Since this temperature is also equal to 0.01¡ÆC, the temperature in kelvins is always equal to 273.15 plus the temperature in degrees Celsius. The kelvin equals exactly 1.8 degrees Fahrenheit. The unit is named for the English mathematician and physicist William Thomson (1824-1907), also known as Baron Kelvin; he is remembered for his pioneering work on the physics of heat. ken a traditional Japanese unit of length comparable to the English fathom. The ken equals 6 shaku, which is about 1.818 meters (5.965 feet). kerat a traditional Middle Eastern unit of length, equal to about 9/8 inch or 2.86 centimeters. The unit has the same Arabic root as the carat. key slang for kilo, meaning kilogram. kibi- (Ki-) a binary prefix meaning 210 = 1024. This prefix, adopted by the International Electrotechnical Commission in 1998, replaces kilo- for binary applications in computer science. Thus 1024 bytes of storage is now a kibibyte, not a kilobyte. The prefix is a contraction of "kilobinary." kilderkin an old British unit of volume equal to 1/2 barrel. Based on the current British barrel, this would be 18 (imperial) gallons, which is about 2.9 cubic feet or 78 liters. Older kilderkins were generally in the range of 16-18 gallons. The word comes from a Dutch word for a small cask. kilo an informal name for a kilogram. kilo- (k-) [1] a metric prefix meaning 1000. The prefix is a modification of chilioi, the Greek word for a thousand. kilo- (k-) [2] in measuring the memory of a computer, the prefix kilo- formerly meant 210 = 1024 instead of 1000. By a 1998 resolution of the International Electrotechnical Commission, the new prefix kibi- (Ki-) replaces kilo- for 210. kilobar (kbar or kb) a metric unit of pressure, used particularly in industrial applications and in geology for measuring high pressures. The kilobar equals 1000 bars, 100 megapascals, or about 14 503 pounds per square inch. (Note: in the investment world a kilobar is not a unit; it is a 1 kg bar of gold, silver, or platinum.) kilobase (kb) a unit of genetic information equal to the information carried by 1000 pairs of the base units in the double-helix of DNA; also used as a unit of relative distance equal to the length of a strand of DNA containing 1000 base pairs. kilobit (kb) a unit of information equal to 1024 bits or 128 bytes, now called a kibibit. kilocalorie (kcal) an ambiguous metric unit of energy. The ambiguity arises because there are two "calories" in common use, identified in this dictionary as the calorie (the small or gram calorie) and the Calorie (the large or kilogram calorie). The term kilocalorie usually means 1000 calories, which is the same as 1 Calorie. In other words, "kilocalorie" is generally used as short form of "kilogram calorie." kilogram (kg) the fundamental unit of mass in the SI and MKS versions of the metric system. The kilogram is defined as the mass of the standard kilogram, a platinum-iridium bar in the custody of the International Bureau of Weights and Measures (BIPM) near Paris, France. Copies of this bar are kept by the standards agencies of all the major industrial nations, including the U.S. National Institute of Standards and Technology (NIST). One kilogram equals exactly 1000 grams, or about 2.204 622 6 pounds. kilogram mole, kilogram molecule, kilogram molecular weight (kgmol or kgmole) various older names for a unit of the amount of a chemical compound. One kilogram mole of a compound is the number of kilograms of the compound equal to the molecular weight of a molecule of that compound measured in atomic mass units. The correct name for this unit is the kilomole (kmol). kilogram of force (kgf) a unit of force equal to the gravitational force on a mass of one kilogram. One kilogram of force equals 9.806 65 newtons, or 2.204 622 6 pounds of force in the traditional English system. Using this unit revives the old confusion between mass and weight, one of the worst features of traditional measurement systems, so it is really a very bad idea. However, kilograms of force have been used rather frequently in engineering and physics. kilojoule (kJ) a common metric unit of work or energy, comparable to the British thermal unit (Btu). In fact, one kilojoule equals approximately 0.947 817 Btu. In other energy units, the kilojoule is also equivalent to 0.238 846 kilocalories, 0.277 778 watt hour, or 737.562 foot-pounds in the traditional English system. kiloliter (kl or kL) a metric unit of volume. The kiloliter is identical to the cubic meter: it equals about 35.3147 ft3, 1.307 95 yd3, 264.17 U.S. gallons, 219.99 British Imperial gallons, 7.497 U.S. bushels, or 6.049 British imperial bushels. kilometer (km) a common metric unit of length or distance. One kilometer equals exactly 1000 meters, about 0.621 371 19 mile, 1093.6133 yards, or 3280.8399 feet. Oddly, higher multiples of the meter are rarely used; even the distances to the farthest galaxies are usually measured in kilometers. The unit is sometimes pronounced with the accent on the first syllable and sometimes on the second. Both are acceptable; there are no official pronunciations for SI units. kilometer per hour (km/h) a common metric unit of speed or velocity. 1 km/h is equal to 5/18 meter per second, 0.621 371 19 mi/hr or 0.911 344 42 ft/sec. kilopascal (kPa) a common metric unit of pressure. One kilopascal equals 1000 pascals (Pa), 10 millibars(mb), or about 0.145 038 pounds per square inch (lbf/in2 or psi), 20.8855 pounds per square foot, 7.502 millimeters of mercury (mm Hg), 0.2953 inches of mercury (in Hg), 4.015 inches of water column (in WC), or 0.3346 foot of head (ft hd). kilopond (kp) another name for the kilogram of force (kgf) (see above). kiloton (kt or kton) [1] an ambiguous unit of mass, ambiguous because it may refer to 1000 U.S. tons (907 185 kilograms), 1000 British tons (1 016 047 kilograms), or 1000 metric tons (1 million kilograms). To avoid this confusion, the metric unit should be written kilotonne. kiloton (kton) [2] a unit of explosive energy equal to the energy released by exploding 1000 U.S. tons (2 million pounds) of TNT. This is about 4.18 terajoules (4.18 x 1012 joules) or 4 billion Btu. kilovolt ampere (kV¡¤A) a common unit of load in power engineering, equal to 1000 volt amperes. kilowatt (kW) a common metric unit of power, equivalent to 1000 watts, about 1.341 022 horsepower, or 737.562 foot-pounds per second. kilowatt hour (kW¡¤h or kw hr) the commercial unit of electric energy. One kilowatt hour represents the amount of energy delivered a rate of 1000 watts over a period of one hour. Since the watt is 1 joule/sec and there are 3600 seconds in an hour, the kilowatt hour is equivalent to exactly 3.6 megajoules of energy, or about 3412.141 Btu, 859.846 (kilogram) Calories, or about 2.655 million foot pounds. kin the Japanese version of the catty, a common weight unit of the Far East. The Japanese identified this unit with a traditional unit equal to about 1.323 pounds or almost exactly 600 grams; this is about 0.75% smaller than the Chinese catty. kip [1] an informal unit of mass, sometimes used by engineers to express the amount of load on a structure. One kip equals 1000 pounds. The name of the unit is an abbreviation of "kilopound." kip [2] an informal unit of force equal to 1000 pounds of force or about 4.4482 kilonewtons. klafter [1] a traditional unit of distance in German-speaking countries, comparable to the English fathom. The Austrian klafter is equal to 1.8965 meters (6.22 feet). In Switzerland the klafter has been brought into the metric system at exactly 1.8 meters (5.9055 feet). This unit is also called the faden. klafter [2] a traditional German unit of volume for stacked firewood, comparable to the English cord. A klafter of wood was generally 1 klafter tall and 1 klafter long, but there was less agreement on its width, that is, the length of the logs. A common width was 3 fusse or 0.5 klafter, making the volume about 3.41 steres (cubic meters) or 0.941 cord by the Viennese definition, 3.34 steres or 0.921 cord in northern Germany. klick, klik see click. knot (kn or kt) a unit of velocity equal to one nautical mile per hour. Knots are customarily used to express speeds at sea, including the speed of the ship as well as the speeds of the wind and of the current. The word comes from the former method of measuring a ship's speed, which involved use of a knotted cord called the log line. With the current definition of the nautical mile, 1 knot equals about 1.1508 miles per hour, exactly 1.852 kilometers per hour, or 0.5143 meters per second. The knot has also been used incorrectly as a name for the nautical mile. koddi a traditional Arabic unit of volume, equal to about 4/3 British imperial gallon or 7.58 liters. koku a traditional Japanese unit of volume, equal to about 180.391 liters (39.68 British imperial gallons or 6.37 cubic feet). kommerzlast German for "commercial load," now interpreted as a metric unit equal to exactly 3 tonnes (about 6613.9 pounds). kon Korean name for the catty. koyan a Far Eastern unit of weight equal to 4000 catty or 5333.33 pounds (8/3 short ton). krina a traditional unit of volume in Bulgaria, now expressed in the metric system as being equal to exactly 20 liters (4.40 British imperial gallons or 5.28 U.S. liquid gallons). kulmet a traditional Latvian unit of volume equal to about 10.93 liters (2.40 British imperial gallons or 2.89 U.S. liquid gallons). A similar Estonian unit, the kulimet, equals about 11.48 liters (2.53 British imperial gallons or 3.03 U.S. liquid gallons). kunitz or Kunitz unit a unit used in biochemistry to describe the concentration or activity of the enzyme ribonuclease, which attacks ribonucleic acid (RNA). Ordinarily RNA absorbs ultraviolet light, so the activity of ribonuclease can be measured by observing the decrease of this absorption. One kunitz is the concentration of the enzyme causing a decrease of 100% in the absorption in a time of one minute, the measurement being made under certain standard laboratory conditions. The unit's name recognizes the Russian-American biochemist M. Kunitz, who proposed the standard test in 1946. kwan a traditional Japanese unit of weight equal to 6.25 kin (see above), which is about 8.27 pounds or 3.75 kilograms. L labor a traditional unit of area in Latin American countries. The labor is equal to the area of a square 1000 varas on a side, or 0.04 legua. Using the Texas standard for the vara (33 1/3 inches), this is equivalent to 177.136 acres or 71.685 hectares. The word labor means work in Spanish, as it does in English. As a unit it represents the area that could be cultivated by a single farmer, somewhat like the old English hide. lakh a traditional unit of quantity in India, equal to 105 or 100 000. This Hindi word is commonly used in English as well; sometimes it means precisely 100 000 and sometimes, like the Greek word myriad (originally 10 000) it simply means "a great number". lambda a metric unit of volume equal to the microliter (¥ìL). The lambda has been used in chemistry in measuring very small samples. The symbol is the lower case Greek letter lambda. lambert (Lb or La) a unit of illumination. One lambert describes the illumination of a surface that emits or reflects one lumen per square centimeter. The lambert is a large unit, and practical measurements tend to be in millilamberts (mLb). The lambert has been replaced for the most part by the SI unit of illumination, the lux; 1 lambert equals 104 lux and 1 millilambert equals 10 lux. (The only difference between "luminance" and "illumination" is the source of the light: if it comes from the surface we speak of "luminance," and if it comes from outside we speak of "illumination." Formerly luminance was measured in lamberts and illumination in phots. In the SI, both concepts are to be measured in lux.) The unit honors the German physicist Johann Lambert (1728-1777), who showed that the illumination of a surface is inversely proportional to the square of the distance from the light source. lane meter a unit of deck area for "roll on/roll off" ships: cargo vessels designed so that containers or other cargo can be rolled on and off the decks of the ship. A lane is a strip of deck 2 meters wide. A lane meter is an area of deck one lane wide and one meter long, that is, 2 square meters. langley a CGS unit of heat transmission equal to one thermochemical calorie per square centimeter, or exactly 41.84 kilojoules per square meter (kJ/m2). Named for the American astronomer Samuel P. Langley (1834-1906), the langley is used to express the rate of solar radiation received by the earth. lap an informal unit of distance used in athletics. A lap is one the length of one trip around a running track. This may vary from track to track, but at the level of serious competition most tracks have a standard length. In English speaking countries this was formerly 1/4 mile (1320 feet or 402.336 meters). Tracks used in most competitions today have a length of exactly 400 meters (1312.34 feet). last an ancient northern European unit used in measuring large quantities, either by mass or volume or both. "Last" is the German word for "load," a meaning which also survives in the English word "ballast." Generally the last is approximately 4000 pounds (about 1800 kilograms) as a mass unit or 80 bushels (about 3 cubic meters) as a volume unit. In the U.S., a last of wool was formerly 12 sacks at 364 pounds per sack, or 4368 pounds (1981.3 kilograms). In the Netherlands, the last is a metric unit of volume equal to exactly 3 cubic meters. In Germany, where the last was previously a volume unit equal to 2.819 cubic meters, it's also a metric unit of mass equal to 2 metric tons (about 4409 pounds). In Britain, a Riga last of timber is 80 cubic feet (2.265 cubic meters) of square-sawn timber. lb, lbf, lbm lb is the traditional English symbol for the pound, derived from the Latin libra, scale. The symbols lbf and lbm are used to distinguish between pounds of force and pounds of mass, respectively. lea a traditional unit of length for yarn, varying with the weight of the fibers in the yarn. Typically a lea of wool is 80 yards (73 meters); a lea of cotton or silk 120 yards (110 meters); and a lea of linen 300 yards (274 meters). In all these cases, a lea is equal to 1/7 hank. A lea is sometimes called a skein. league a traditional unit of distance. Derived from an ancient Celtic unit and adopted by the Romans as the leuga, the league became a common unit of measurement throughout western Europe. It was intended to represent, roughly, the distance a person could walk in an hour. The Celtic unit seems to have been rather short (about 1.5 Roman miles, which is roughly 1.4 statute miles or 2275 meters), but the unit grew longer over time. In many cases it was equal to 3 miles, using whatever version of the mile was current. At sea, the league was most often equal to 3 nautical miles, which is 1/20 degree [2], 3.45 statute miles, or about 5556 meters. The British definition of the league is 3 statute miles (about 4828 meters) on land or 3 nautical miles at sea. In the U.S., leagues have never been used on land and the only definition is 3 nautical miles. However, many occurrances of the "league" in English-language works are actually references to the Spanish league (the legua), the Portuguese league (legoa) or the French league (lieue). For these units, see below on this page. leap a traditional Welsh unit of distance equal to 6 feet 9 inches or 2.0574 meters. legoa the Portuguese league, equal to 3 milhas (Portuguese miles). This is equal to about 3.836 statute miles or 6174.1 meters. legua [1] the Spanish league. The traditional legua is equal to 5000 varas, which is close to 2.6 miles or 4.2 kilometers. Using the Texas definition of the vara, the legua is 2.6305 miles, 13889 feet, or 4233.4 meters. Using the traditional Spanish definition, it would be 2.597 miles, 13712 feet, or 4179.4 meters. Technically, this unit was abolished by Philip II in 1568, but it remained in rather wide use, especially in the Americas. During the late 18th and early 19th centuries, a league of 8000 varas (4.15 miles or 6680 meters) was legal in Spain. At sea, Spanish sailors used the usual marine league (3 nautical miles or 5556 meters) or Phillip's "geographical" league of 1/17.5 degree (3.429 nautical miles or 6350.5 meters). legua [2] a traditional Spanish unit of area equal to one square legua [1]. In Spanish-speaking Latin America and the southwestern states of the U.S. land was customarily measured in leguas, with 1 legua equal to 25 labors (see above) or 25 million square varas. Using the Texas definition of the vara as the starting point, the legua is 4428.4 acres, 6.919 square miles, 1792 hectares, or 17.92 square kilometers. A slightly larger figure, 4439 acres (1796 hectares), is used in California. Larger sizes, between 1800 and 1900 hectares, were formerly used in some parts of South America. length (lg) an informal unit of distance. The distance between competitors in horse races, boat races, and similar situations is naturally expressed in lengths, with one length equal to the average length of a horse, boat, etc. In horse racing, the length is actually interpreted as a unit of time equal to 1/5 second: the number of lengths by which one horse beats another is computed as 5 times the difference in their times as measured in seconds. lethal dose (LD) a measure used in pharmacology to express the percentage of a population killed by a dose of the substance being studied. The measurement is often given as a subscript. For example, the potency of a drug or pesticide is commonly expressed by stating the size of the LD50 dose: the amount of the substance which kills 50% of the test population. li a traditional unit of distance in China. Although current Western dictionaries usually describe the li as being approximately 1/3 mile or 500 meters, the late imperial governments of China used a li of 1800 ch'ih, which is 2115 feet, about 0.401 mile, or 644.65 meters. The well-known Confucian proverb actually says "a journey of a thousand li begins with a single step." liang a traditional Chinese weight unit. During the European colonial period the liang was equal to 1/16 catty, 1/12 pound, or about 37.8 grams; this made it the same as a tael. In modern China, the liang equals 1/10 jin or 10 qian; this is exactly 50 grams (1.7637 ounces). libra (lb) a traditional unit of weight in Italian, Spanish, and Portuguese speaking countries. The libra was the Roman unit from which the English pound is descended; the symbol "lb" for the pound comes from this unit. The Roman libra contained only 12 unciae (ounces) and was about 0.722 English pound. The traditional Italian libra was of similar size. The Spanish and Portuguese units are larger, generally in the range from 1.011 to 1.016 English pound (very close to 460 grams). The Spanish libra equals 16 onzas, and the Portuguese libra equals 16 oncas. The word "libra" is sometimes used now for the kilogram, a much larger unit. lieue the French league. A variety of lieue units were used for land measurement in France, but generally these units were around 2.4-2.5 statute miles in length. In the 18th century, the legal unit was the lieue de poste, defined to equal 2000 toises or 2 milles (2.4221 miles or 3898 meters). In metric France the lieue is now considered to equal exactly 4 kilometers (2.4855 miles). See league (above). At sea, the lieue was often taken to equal 1/25 degree [2] or 2.4 nautical miles (4445 meters or 2.7619 miles); this unit was gradually replaced by the internationally recognized 3 nautical miles (5556 meters or 3.452 miles). light year (ly) a unit of distance used in astronomy. One light year is the distance that light travels in one year through a vacuum (and, of course, most of the Universe is close to being a vacuum). Since light travels at 299 792 458 meters per second, and there are 31 556 925.9747 seconds in a year, one light year equals 9.460 528 405 x 1015 meters, or 9.460 528 405 trillion kilometers. One light year is approximately 5.880 trillion miles. ligne a traditional unit of distance in French speaking countries, equal to 1/12 pouce (French inch) and corresponding closely to the English line [1]. The Swiss ligne is used throughout the world by watchmakers; it equals about 2.256 millimeters (0.0888 inch) and is divided into 12 douziemes. In English the ligne is usually pronounced "lean" by those who don't hold to the French pronunciation. line (li) [1] a traditional unit of distance, equal to 1/12 inch (about 2.1167 millimeters). For measuring the thickness of buttons, there is also a smaller line equal to 1/40 inch (0.635 millimeter). The line is called the ligne (see above) in French, the linea in Spanish, the linie in German, and the liniya in Russian. line (li) [2] a former name for the maxwell, the CGS unit of magnetic flux. The unit was called the line because magnetic fields were traditionally represented by lines depicting the direction of the field; the idea was to quantify the strength of these lines. This is a small unit, so fields were often measured in megalines; one megaline is equal to 0.01 weber. linear foot (or "lineal" foot) terms used loosely to describe a one-foot length of any long, narrow object. The correct term is linear foot; the word "lineal" refers to a line of ancestry, not to length. Boards, pipes, and fencing are typical objects measured and sold by the linear foot. In the moving industry, a linear foot is a one-foot length of a moving van, usually a volume of about 72 cubic feet (roughly 2 cubic meters). Occasionally the term "linear foot" is used as an alternate name for the board foot, but this is appropriate only if the board is 12 inches wide. link a traditional unit of distance used by surveyors, equal to 0.01 chain. In Britain, one link is exactly 0.66 feet, or 7.92 inches, or approximately 20.12 centimeters. In the U.S., where 100-foot chains are more common, the link is the same as the foot. liter or litre (L or l) the common metric unit of volume. The liter was originally defined to be the volume occupied by a kilogram of water, and the gram as the mass of a cubic centimeter of water. This would make the liter equal to exactly one cubic decimeter, that is, to the volume of a cube 0.1 meter (or 10 centimeters) on a side. Unfortunately, the physical objects constructed to represent the meter and kilogram disagreed slightly. As measured by the standard meter and standard kilogram, the standard liter turned out to be about 1.000 028 cubic decimeters. This discrepancy plagued the metric system for a long time. In 1901 an international congress accepted the discrepancy and formally defined the liter to be exactly 1.000 028 dm3. No one was particularly happy with such an awkward definition, so in 1964 the CGPM repealed the definition and abolished the liter as an SI unit. The SI now requires that volumes be measured in cubic meters or power-of-ten multiples thereof, not in liters. The CGPM recommends that the liter be retained only as an informal name for the cubic decimeter in situations where small errors are not important. In its new guise as the cubic decimeter, the liter is approximately 61.023 744 cubic inches. Compared to the customary volume units, the liter is a little more than a U. S. liquid quart (1.056 688 qt) but a little less than a U. S. dry quart (0.908 08 qt) or a British imperial quart (0.879 89 qt). Its name comes from a French volume unit, the litron, which was in turn derived from the Latin litra. Both the lower case letter l and the upper case L are accepted as symbols for the liter, but the U.S. Department of Commerce specifies that L be used, at least by businesses, to avoid confusion with the numeral 1. The unit is spelled liter in the U.S. and litre in Britain; there are many other spellings in various languages (see Spelling of Metric Units). liter atmosphere (L atm) a unit of work or energy used in the study of confined gases. The behavior of gases is described, to a first approximation, by the ideal gas law PV = nRT. The ideal gas law is really an energy equation in which the left hand side, pressure P (in atmospheres) times volume V (in liters), measures the potential energy in the confined gas. One liter atmosphere is equal to 101.325 joules, 0.09605 Btu or 74.73 foot pounds. livre a traditional unit of weight in French speaking countries and in Greece. The livre corresponds to the English pound and to the Spanish libra (see above). The livre is divided into 2 marcs or into 16 onces. The French livre varied from market to market, but the official standard from about 1350 to the introduction of the metric system was the livre poids de marc or livre de Paris of 489.5 grams (1.079 English pounds). In modern France, the livre is used as a metric unit equal to exactly 500 grams or 0.5 kilogram (1.1023 pounds). The traditional Greek livre is also about 500 grams. load an informal unit of volume. In landscaping and some construction trades a load often means a cubic yard (0.764555 m3). In ordinary language in the U.S., a load often means the volume of a pickup truck, a varying unit. long hundredweight the British hundredweight, equal to 112 pounds. long ton the traditional British ton, equal to 2240 pounds. lot a traditional unit of weight in German speaking countries, equal to about 1/2 ounce or 15 grams. "Lot" is the German word for a lead plumbbob, so the unit represents a small lead weight. The plural is lote. lpi abbreviation for lines per inch, a unit used to state the resolution of display devices (such as television or computer monitor screens), or to state the line spacing of printed pages. lug [1] an old English name for a rod (5.5 yards or 5.0292 meters). In some parts of England this unit represented a longer rod of 7 yards (6.4008 meters), a unit also called the great lug. lug [2] a shallow box or crate for produce such as cherries, grapes, or peaches. The size of a lug varies with the item it contains. Typical lugs hold about 16-28 pounds (7-13 kilograms) of produce in a volume of roughly 1/3 bushel (about 12 liters). This unit seems to be particularly common in produce markets in the midwestern U.S. lumen (lm) the SI unit for measuring the flux of light being produced by a light source or received by a surface. The intensity of a light source is measured in candelas. One lumen represents the total flux of light emitted, equal to the intensity in candelas multiplied by the solid angle in steradians (1/(4¡¤pi) of a sphere) into which the light is emitted. Thus the total flux of a one-candela light, if the light is emitted uniformly in all directions, is 4¡¤pi lumens. lumen hour (lm h) a unit of quantity of light, equal to one lumen of light flux continued for one hour. The lumen second (lm s) is defined similarly. luster, lustre, lustrum a traditional unit of time equal to 5 years. In ancient Rome the Lustrum was ceremony of expiation and purification for the whole population of the city, carried out every 5 years after the completion of the census. The use of luster or lustrum as a unit of time in English was fairly common in well-educated circles as long as "well-educated" meant classically educated; the unit has pretty much disappeared today. lux (lx) the SI unit for measuring the illumination of a surface. One lux is defined as an illumination of one lumen per square meter. The difference between these light units may seem a bit subtle. Here's a review: the intensity of a light source is measured in candelas; the total light flux transmitted is measured in lumens (1 lumen = 1 candela¡¤steradian); and the amount of light received per unit area is measured in lux (1 lux = 1 lumen/square meter). One lux is equal to approximately 0.09290 foot candle. M M [1] informal abbreviation for million in expressions where the base unit is understood, as in "500M hard drive" (500 megabytes). In chemistry, M is the symbol for "molar" (see below). M [2] the Roman numeral 1000, sometimes used in symbols to indicate a thousand, as in Mcf, a traditional symbol for 1000 cubic feet. This use of the letter M is very confusing and should be scrapped. mace a traditional Chinese unit for weighing precious metals, especially silver. In the colonial period, the mace was considered to equal 0.1 tael or liang; this would be 2/15 ounce or about 3.78 grams. mach (M or Ma) a measure of relative velocity, used to express the speed of an aircraft relative to the speed of sound. The name of the unit is placed before the measurement. Thus "mach 1.0" is the speed of sound, "mach 2.0" is twice the speed of sound, and so on. The actual speed of sound varies, depending on the density and temperature of the atmosphere. (At 0¡ÆC and a pressure of 1 atmosphere the speed of sound is about 1088 ft/sec, 331.6 m/sec, or 741.8 mi/hr). The mach speed is important to the control of an aircraft, especially at speeds close to or exceeding mach 1.0. The unit is named for the Austrian physicist Ernst Mach (1838-1916). magnitude (mag) [1] a unit traditionally used in astronomy to express the apparent brightness of stars, planets, and other objects in the sky. For centuries, the brightest stars were said to be of the "first magnitude," with fainter ones of the "second magnitude" and so on down to "sixth magnitude" for the faintest stars visible to the unaided eye. When it became possible to measure stellar brightnesses precisely, it was discovered that stars of a given traditional magnitude were roughly 2.5 times brighter than stars of the next magnitude. Astronomers agreed to define the magnitude scale so that a difference of exactly 5.0 mag corresponds to a brightness difference of exactly 100 times. A difference of 1.0 mag then corresponds to a brightness difference of the fifth root of 100 or about 2.512 times. The scale is very unusual because it is upside down: brighter stars have lower, not higher magnitudes, in keeping with the historical origin of the scale. The zero point (0.0 mag) is set arbitrarily so that the stars historically listed as "first magnitude" have magnitude measurements of 1.5 mag or brighter. magnitude (mag) [2] a unit used in earth science to measure the intensity of earthquakes. Geologists actually use several scales to measure earthquake intensity, but the one best known to the public is the Richter magnitude scale, developed in 1935 by Charles F. Richter (1900-1985) of the California Insitute of Technology. The Richter magnitude is computed from the measured amplitude and frequency of the earthquake's shock waves received by a seismograph, adjusted to account for the distance between the observing station and the epicenter of the earthquake. An increase of 1.0 in the Richter magnitude corresponds to an increase of 10 times in the amplitude of the waves. The most powerful earthquakes recorded so far had magnitudes of about 8.5. The Richter magnitude measures the intensity of the earthquake itself, not the intensity of the earthquake's effects: the effects also depend on the depth of the earthquake, the geology of the area around the epicenter, and many other factors. Earthquake effects are rated using the Mercalli scale (see below). magnum a traditional unit of volume for wine, generally equal to 2 bottles. This is now exactly 1.5 liters (about 2.114 U.S. quarts). mahnd a traditional Arab weight unit, equal to about 2.04 pounds or 907 grams. man hour a common unit of labor, equal to the work of one person for one hour. The less restrictive term person hour is gradually coming into use. manpower an informal unit of power equal to 0.1 horsepower or about 74.57 watts. The unit seems to have been invented by American engineers. manzana a traditional unit of land area in Central America. The manzana is the area of a square 100 varas on a side; it thus varies according to the length of the vara. The Costa Rican manzana equals 0.698 896 hectares or about 1.727 acres. Very similar units are used in Honduras and Nicaragua. The word manzana means an apple. The unit is probably related to manzanar, orchard. marathon a traditional unit of distance used in athletics. The length of a marathon is exactly 26 miles 385 yards, or about a millimeter less than 42 194 meters. A story is often told that the marathon is the exact distance run by Pheidippides in 490 BC to bring to Athens the news of the Greek victory over the Persians at the Battle of Marathon. Unfortunately, the actual distance from Marathon to Athens is only about 36.75 kilometers. In fact, 26 miles 385 yards is simply the distance run in the marathon event at the 1908 Olympics in London. The marathons at the Olympic Games were of varying lengths until the Paris Olympics of 1924, when the International Olympic Committee fixed the 1908 London distance as official. marc, marco, or mark traditional units of weight in various countries of Western Europe. In each country the unit equals 1/2 the unit corresponding to the English pound. Thus the French marc equals 1/2 livre, 8 onces or about 244.75 grams; the Spanish marco equals 1/2 libra or about 230 grams; the German mark equals 1/2 pfund or about 280.5 grams; and the English mark equals 8 ounces or 226.8 grams. The English unit was used almost entirely for measuring precious metals. mas symbol for milliarcsecond, a unit of angular measure commonly used in astronomy. maund a traditional unit of weight in India and throughout South Asia. The maund varied considerably, but during the period of British rule in India it was standardized at about 82.286 pounds or 37.3242 kilograms. The maund is divided into 40 seers. Since 1980, Pakistan has used a metric version of the maund equal to exactly 40 kilograms (88.185 pounds); this would make the seer equal to the kilogram. maxwell (Mx) a CGS unit of magnetic flux, equal to 10-8 weber. In a magnetic field of strength one gauss, one maxwell is the total flux across a surface of one square centimeter perpendicular to the field. This unit was formerly called the line [2]. The newer name honors the British physicist James Clerk Maxwell (1831-1879), who presented the unified theory of electromagnetism is 1864. mc- alternate symbol for micro- (see below). This prefix is often seen in the symbol mcg for the microgram. Occasionally, mc has been used as a symbol for the micron (see below). MBF or MBM traditional symbols for 1000 (not one million) board feet, a unit of volume for timber equal to 250/3 = 83.333 cubic feet or 2.360 cubic meters. "BM" stands for "board measure." MCF a traditional symbol for 1000 (not one million) cubic feet, a unit of volume equal to about 28.317 cubic meters. mease a unit of quantity formerly used by fishermen. The mease equals the number of herring in a basket, roughly 620. mebi- (Mi-) a binary prefix meaning 220 = 1 048 576. This prefix, adopted by the International Electrotechnical Commission in 1998, replaces mega- for binary applications in computer science. The prefix is a contraction of "megabinary." meg informal contraction of "megabyte," used in computer science. mega- (M-) [1] a metric prefix meaning 106, or one million. (The form meg- is used before a vowel, as in megohm for one million ohms.) The prefix has also become common in ordinary language, meaning "very large," as in megabucks or megadose. This agrees with the derivation of the prefix from the Greek word for large, megas. mega- (M-) [2] in measuring the storage capacity of a computer, the prefix mega- formerly meant 220 = 1 048 576 instead of an even one million. By a 1998 resolution of the International Electrotechnical Commission, the new prefix mebi- (Mi-) replaces mega- for 220. megabase (Mb) a unit of genetic information equal to the information carried by 1 million pairs of the base units in the double-helix of DNA; also used as a unit of relative distance equal to the length of a strand of DNA containing 1 million base pairs. In humans, one megabase corresponds approximately to a gene separation of one centimorgan. megabyte (MB) this unit of information is very common in the computer world, but it is poorly defined. Often it means 1 000 000 bytes, but sometimes it means 220 = 1 048 576 bytes. As if that weren't confusing enough, the 1.44 megabytes stored on "high density" floppy disks are actually megabytes of 1 024 000 bytes each. This uncertainty is a major reason for the recent decision of the International Electrotechnical Commission to establish new binary prefixes for computer science. megajoule (MJ) a common metric unit of work or energy. The megajoule equals one million joules, which is approximately 737 562 foot pounds, 947.8170 Btu, 238.846 (kilogram) Calories, or 0.277 778 kilowatt hours. megalerg a CGS unit of energy equal to 106 ergs or 0.1 joule (0.073 756 foot pound). The "l" was added to "mega-erg" to make the unit pronounceable. megaline a metric unit of magnetic flux, equal to one million lines [2] or 0.01 weber. megaparsec (Mpc) the longest distance unit in common use, the megaparsec is used by astronomers studying the most distant quasars and galaxies. One megaparsec equals 2.36 million light years or 30.857 x 1018 kilometers. megapascal (MPa) a common metric unit of pressure. One megapascal equals 10 bars or approximately 145.038 pounds per square inch (lbf/in2 or psi) or 20 885.5 pounds (10.443 U.S. tons) per square foot. megaton (Mt) a unit of energy used for measuring the energy of an explosion, especially a nuclear explosion. One megaton is the amount of energy released by the explosion of one million (short) tons of TNT. This is approximately 4.18 x 1015 joules (4.18 petajoules) or roughly 4 trillion Btu. megawatt (MW) a common metric unit of power. One megawatt is equal to one million watts or about 1341.02 horsepower or 947.817 Btu per second. megawatt hour (MW¡¤h) a metric unit of energy, especially electrical energy. One megawatt hour equals exactly 3.6 gigajoules (GJ), about 3.412 million Btu, or about 2.655 billion foot pounds. meile a traditional distance unit in German speaking countries, the meile is much longer than the mile units of western Europe. Typically the meile was equal to 4000 klafters (fathoms) or 24 000 fusse (German feet). In Austria this came to 7586 meters (4.714 miles); in northern Germany it was 4.6805 miles or 7532.5 meters. A version of the meile called the geographische meile was defined to equal exactly 4 (Admiralty) nautical miles (24 320 feet, 4.6061 miles, or 7412.7 meters). Mercalli intensity scale an empirical scale for rating the effects of an earthquake, as opposed to its strength (see magnitude [2] above). Mercalli estimates are stated as Roman numerals (I-XII) to avoid confusion with magnitude estimates on the Richter scale. The scale is named for the Italian geologist Giuseppe Mercalli, who devised the first version in 1902; the modified version used in the U.S. and Canada was developed by Charles F. Richter in 1956. -merous an ending meaning "-parted," added to a number to create an adjective. Thus "8-merous" means "having 8 parts." The suffix, frequently used by botanists, is derived from the Greek meros, "part." mesh a traditional unit used to measure the fineness of woven products such as fishing nets, fencing fabric, window screening, etc., equal to the number of strands per inch. met a unit of metabolism. Metabolism, the sum of all the processes going on in the body to sustain life, is measured in units of power expended per unit of body surface area. One met is the metabolism of a seated, resting person, equal to about 58.15 watts per square meter (W/m2) or 13.89 calories per second per square meter (cal/m2s) regardless of the person's size. Measurements of human metabolism generally fall in the range 0.8-3.0 met, although athletes can achieve 10 met or more. meter or metre (m) the fundamental metric and SI unit of distance. Originally, the meter was defined to be one ten-millionth of the distance between the Equator and the North Pole. Because the Earth is difficult to measure (not to mention the many irregularities in its shape!), this is not a practical definition. For a long time, the meter was the length of an actual object, a bar kept at the International Bureau of Weights and Measures in Paris. In recent years, however, the SI fundamental units (with one exception) have been redefined in abstract terms which can (in principle, at least) be reproduced to any desired level of accuracy in a well-equipped laboratory. For the meter, the 17th General Conference on Weights and Measures in 1983 adopted this "simple" definition: the meter is defined as that distance which makes the speed of light in a vacuum equal to exactly 299 792 458 meters per second. The speed of light in a vacuum is one of the fundamental constants of nature. Experiments previously made to measure the speed of light are now reinterpreted as measurements of the meter instead. The meter is equal to approximately 1.093 613 3 yards, 3.280 840 feet, or 39.370 079 inches. Its name comes from the Latin metrum and the Greek metron, both meaning "measure." The unit is spelled meter in the U.S. and metre in Britain; there are many other spellings in various languages (see Spelling of Metric Units). methuselah a large wine bottle holding about 6 liters, 8 times the volume of a regular bottle. metric carat the current internationally-recognized carat, equal to exactly 200 milligrams. metric grain a unit of mass sometimes used by jewelers, equal to 50 milligrams or 1/4 carat. metric horsepower a unit of power, defined to be the power required to raise a mass of 75 kilograms at a velocity of 1 meter per second. This is approximately 735.499 watts or 0.986 32 horsepower. The unit is also known by its French name cheval vapeur or its German name pferdestarke. metric hundredweight an informal unit of mass equal to 50 kilograms or approximately 110.231 pounds. This is quite close to the traditional British hundredweight of 112 pounds. This unit is also known by its German name, the zentner, or (in English) the centner. metric mile an informal unit of distance used mostly in athletics. The metric mile is equal to 1500 meters or 1.5 kilometers (approximately 0.932 057 statute mile or 4921.26 feet). In U.S. high school competition the term is sometimes used for a race of 1600 meters (0.994 194 miles or 5249.34 feet). metric quintal a unit of mass equal to 100 kilograms or about 220.462 pounds. See quintal for a more complete description. metric slug see TME. metric ton (MT) an alternate name for the tonne. In the United States, the Department of Commerce strongly recommends that the tonne be called the metric ton to distinguish it clearly from the traditional American ton. The proper symbol for the unit is simply t. Mgd an abbreviation for millions of gallons per day (Mgal/d), a unit used in reservoir management to express the rate at which water is withdrawn, or could be withdrawn, for drinking or for some other purpose. 1 Mgd equals approximately 3.785 43 megaliters per day, or 3785.43 cubic meters per day, or 133 681 cubic feet per day. mg/dl symbol for milligram per deciliter, the unit used in medicine to measure the concentration of cholesterol and other substances in the blood. 1 mg/dl equals 0.01 grams per liter (g/L). mg/kg symbol for milligram per kilogram, a unit used in medicine to measure dosage rates. 1 mg/kg is equivalent to 10-6 g/g or 1 part per million based on the patient's body weight. mho a unit of electric conductance, equal to 1 divided by the impedance of a circuit as measured in ohms. This makes the mho the same as the siemens, except that the mho is used in circuits in which current is retarded by self-inductance as well as by resistance. In case you didn't notice, "mho" is "ohm" spelt backwards. mic an informal name for the microgram, pronounced "mike." micro- (¥ì- or mc-) a metric prefix meaning 10-6 (one millionth). The prefix comes from the Greek prefix mikro-, meaning small. The prefix is abbreviated mc- when the Greek letter mu (¥ì) is not available. micron (¥ì) a metric unit of distance equal to one millionth of a meter. "Micron" is simply a shorter name for the micrometer. In 1968 the CGPM decided to drop the micron as an approved unit and recommend that micrometers be used instead. Microns, however, are still in common use. miglio the traditional Italian mile. The miglio equals 1628 yards, which is 0.925 English mile or about 1488.6 meters. This is 32 yards (29.3 meters) shorter than the classical Roman mile. miil or mijl alternate spellings for the Scandinavian mil [4] (see below). mil [1] a unit of distance equal to 0.001 inch: a "milli-inch," in other words. Mils are used, primarily in the U.S., to express small distances and tolerances in engineering work. One mil is exactly 25.4 microns, just as one inch is exactly 25.4 millimeters. In Britain this unit is usually called the thou. mil [2] a unit of angle measure, used in the military for artillery settings. During World War II the U. S. Army often used a mil equal to 1/1000 of a right angle, 0.1 grad, or 0.09¡Æ. More recently, various NATO armies have used a mil equal to 1/1600 right angle, or 0.05625¡Æ. In Britain, an angular mil is often understood to mean 0.001 radian, which is about 0.0573¡Æ. (With this last definition, an object one meter wide subtends an angle of one mil at a distance of one kilometer.) mil [3] an informal British unit of volume equal to the milliliter (mL). mil [4] in Scandinavia, the "mil," pronounced like "meal" in English, is a traditional distance unit considerably longer than Roman or English miles. In Denmark, the traditional mil was 24 000 Danish feet, which is 4.6805 miles or 7.5325 kilometers (this is the same as the north German meile; see above). The Danish mil has sometimes been interpreted as exactly 7.5 kilometers (4.6603 miles). In Sweden, the traditional mil was 36 000 Swedish feet, which is 6.641 miles or 10.687 kilometers. In Sweden, Norway, and Iceland, the mil (mila in Iceland) is now interpreted as a metric unit equal to exactly 10 kilometers (6.2137 miles). mil [5] an alternate spelling of the mill (see below). mile (mi) a traditional unit of distance. The word comes from the Latin phrase milia passuum, "thousands of paces," because originally a mile was the distance a Roman legion could march in 1000 paces (right foot, left foot that's one pace). The Roman mile appears to have been equal to about 4980 feet or 1517 meters. Miles of approximately this length were used throughout Western Europe. In 1592 the British Parliament defined the statute mile to be 320 rods, 1760 yards or 5280 feet. This unit replaced various miles previously used in Britain, including the traditional Scottish mile of 1980 yards and the Irish mile of 2240 yards. In metric terms, 1 statute mile is exactly 1609.344 meters. In athletics, 1500 meters is sometimes called a metric mile. See also nautical mile. miles per gallon (mi/gal or mpg) the unit customarily used in the United States to measure the fuel consumption rate of motor vehicles. 1 mile per gallon equals about 0.4252 kilometers per liter. In most other countries, however, the usual measure of fuel consumption is liters per 100 kilometers; m miles per U.S. gallon is equal to 235.215/m liters per 100 kilometers. Similarly, p miles per British imperial gallon is equal to 282.481/p liters per 100 kilometers. miles per hour (mi/hr or mph) a traditional unit of velocity. One mile per hour equals exactly 22/15 feet per second, approximately 1.609 kilometers per hour, or about 0.447 meters per second. milha the traditional Portuguese mile, one of the "longest miles" of all at 2282.75 yards (1.297 statute miles or 2087.3 meters). military pace another name for a step. In the U.S. Army, the military pace is defined to be exactly 30 inches (76.2 centimeters) for ordinary "quick time" marching and 36 inches (91.44 centimeters) for double time marching. The same definitions are generally used by marching bands. mill a unit of quantity or of proportion, equal to 0.001. When Congress established the U. S. monetary system in 1791, it provided for 10 mills to the cent and 100 cents to the dollar; thus the mill was an amount of money equal to $0.001. Although the mill is obsolete now as a monetary unit, it has come to represent a one thousandth part as a proportion. Many towns in the United States set their property tax rates in mills, for example. milla the traditional Spanish mile, equal to 5000 pies (Spanish feet) or 8 estadios. This is about 1392 meters, 4567 feet, or 0.865 statute mile. mille the traditional French mile, equal to 1000 toises. This is equal to about 6394.4 feet, 1.211 statute mile, or 1949 meters. millennium a traditional unit of time equal to 1000 years. milli- (m-) a metric prefix meaning 0.001 (one thousandth). The prefix was coined from the Latin number mille, one thousand. milliard [1] a unit of quantity equal to 109, which is what Americans call a billion. See Using Numbers and Units for more on the "billion" controversy. milliard [2] a unit of volume used by engineers to describe a large quantity of water. One milliard equals one cubic kilometer, which is 1 billion (109) cubic meters or about 810 767 acre feet. millibar (mb) a common metric unit of atmospheric pressure, equal to 0.001 bar, to 100 pascals, to 1000 dynes/cm2, to about 0.0295 inches of mercury, or to about 0.014 504 lb/in2. A millibar is the same thing as a hectopascal (hPa), which is the correct SI unit to use for atmospheric pressure. However, many meteorologists continue to use millibars. milliequivalent (mEq or meq) a unit of relative amount of substance commonly used in chemistry. One mEq equals 0.001 equivalent weight. millier a former name for the tonne or metric ton. This name was used in Britain to avoid confusion with the British long ton. It is now obsolete. milligram (mg) a common metric unit of mass. One milligram equals approximately 0.015432 grain or 35.274 x 10-6 ounce. millihg an informal name (pronounced "millig") for the millimeter of mercury (see below). milliliter (ml or mL) a common metric unit of volume. One milliliter equals one cubic centimeter or approximately 0.061 023 7 cubic inch or 16.231 U.S. minims (see below). millimeter (mm) a common metric unit of distance. One millimeter equals about 0.039 370 inch or 39.370 mils. millimeter of mercury (mm Hg) a unit of pressure equal to the pressure exerted at the Earth's surface by a column of mercury 1 millimeter high. When a traditional mercury barometer is used, the pressure is read directly as the height of the mercury column in millimeters. One millimeter of pressure is equivalent to approximately 0.03937 in Hg, 0.01933 lb/in2, 1.333 millibars, or 133.3 pascals. In medicine, blood pressure is traditionally recorded in mm Hg. (In engineering, the millimeter of mercury is often replaced by the torr, the two units being equal to within 1 part per million.) In the symbol mm Hg, Hg is the chemical symbol for mercury. millimeter of water (mmH2O or mm WC) a unit of pressure equal to the pressure exerted at the Earth's surface by a column of water 1 millimeter high. This is a small pressure, about 9.8067 pascals, 0.098 067 millibars, 0.03937 inch of water, or 0.204 pounds per square foot. millimicron (m¥ì) a former metric unit of distance equal to 0.001 micron or 10-9 meter. The millimicron has been replaced by its equivalent, the nanometer (nm). milline a traditional unit of advertising. One milline equals the width of a line of "agate" type (5.5 points, or about 2 mm) times the width of a column times one million copies of the publication. milliosmole (mOsm) a unit of osmotic pressure, equal to 0.001 osmole, commonly used in biology and medicine. millisievert (mSv) a unit commonly used to measure radiation dose. One millisievert equals 0.001 sievert or 0.1 rem. miner's inch a traditional unit of water flow in the western United States. The unit originally represented streamflow through an opening one inch square with a specified water pressure. In Idaho, Kansas, Nebraska, New Mexico, North and South Dakota, Utah, and Washington the miner's inch is legally defined to equal 9 gallons per minute or 1.2 cubic feet per minute (about 34.07 liters per minute). In Arizona, California, Montana, Nevada, and Oregon the definition is 1.5 cubic feet per minute (42.48 L/min). In Colorado, the legal equivalent is 1.5625 cubic feet per minute (44.25 L/min). See also water inch. minim a traditional unit of volume used for very small quantities of liquids. In pharmacy, the term drop traditionally meant the same thing as 1 minim. The minim is defined to be 1/60 fluid dram or 1/480 fluid ounce. The U. S. minim is equal to about 0.003 760 cubic inch or 0.061 610 milliliter, while the British minim is equal to about 0.003 612 cubic inch or 0.059 194 milliliter. As you might guess, the word comes from the Latin minimus, small. minute (min or ') [1] a unit of time equal to 60 seconds or to 1/60 hour. The word comes from the Latin word minutus, meaning "small." The SI specifies min as the symbol for the time unit and ' as the symbol for the arcminute (see below). minute (' or m) [2] a unit of angular measure equal to 60 arcseconds or to 1/60 degree. This unit is often called the arcminute to distinguish it from the minute of time. There are 21 600 arcminutes in a circle. minute (' or m) [3] a unit of longitude used in astronomy. Astronomers measure longitude (or "right ascension", as they call it) in time units by dividing the equator into 24 hours instead of 360 degrees. This makes 1 minute of longitude or right ascension equal to 15 arcminutes. mips a unit of computing power equal to one million instructions per second. An "instruction" is a single program command to the computer's central processor. In a particular computer, there is a definite relationship between the rate at which instructions are processed, in mips, and the "clock speed" of the processor, measured in megahertz (MHz). However, this relationship varies considerably between computers, so it is usually not meaningful to compare the mips rates of dissimilar machines. mkono a traditional unit of distance in East Africa, standardized under British rule as 1/2 yard (18 inches, or 45.72 centimeters). MM an abbreviation for one million, seen in a few traditional units such as the following entry. The abbreviation is meant to indicate one thousand thousand, M being the Roman numeral 1000. However, MM actually means 2000, not one million, in Roman numeration. MM Btu traditional symbol for one million Btu (about 1.055 057 gigajoules (GJ)), a unit used widely in the energy industry. This unit is also called the dekatherm. MM scfd symbol for one million standard cubic feet per day, the customary unit for measuring the production and flow of natural gas. "Standard" means that the measurement is adjusted to standard temperature (0 ¡ÆC) and pressure (1 atmosphere). -mo a "unit" traditionally used in printing to describe the page size of a book or other publication. In traditional printing, large sheets are printed, folded, and then cut to manufacture the book. After the cut is made, the sheet has been divided into a certain number of "leaves." Each leaf, folded at the spine of the book, comprises two pages front and back. When sheets were cut to form 4, 8, or 12 leaves, the resulting pages were described as quarto (4to), octavo (8vo) or duodecimo (12mo), respectively. Later, the suffix -mo from duodecimo was made into a suffix which can be attached to any number to indicate the number of leaves per sheet; thus 16mo indicates 16 leaves per sheet. Mohs hardness scale a 1-10 scale for estimating the hardness of a mineral, introduced by the German geologist Friedrich Mohs in 1812. To apply the scale, one attempts to scratch the mineral with standard minerals assigned hardness numbers as follows: diamond 10, corundrum 9, topaz 8, quartz 7, orthoclase 6, apatite 5, flourite 4, calcite 3, gypsum 2, and talc 1. If, for example, the mineral is scratched by quartz but not by orthoclase, then its hardness is between 6 and 7. molal (m), molar (M) these notations, traditionally used by chemists to describe the concentration of chemical solutions, often appear to be units of measurement. It's easy to get them confused. The term "molal" describes the concentration of a solution in moles per kilogram of solvent (mol/kg), while "molar" describes a concentration in moles per liter (mol/L). A solution described as 1.0 ¥ìM has a concentration of 1.0 ¥ìmol/L. These units are not approved by the General Conference on Weights and Measures. Their use is declining, but still substantial. mole (mol) the SI fundamental unit of the amount of a substance (as distinct from its mass or weight). Moles measure the actual number of atoms or molecules in an object. An alternate name is gram molecular weight, because one mole of a chemical compound is the same number of grams as the molecular weight of a molecule of that compound measured in atomic mass units. The official definition, adopted as part of the SI system in 1971, is that one mole of a substance contains just as many elementary entities (atoms, molecules, ions, or other kinds of particles) as there are atoms in 12 grams of carbon-12 (carbon-12 is the most common atomic form of carbon, consisting of atoms having 6 protons and 6 neutrons). The actual number of "elementary entities" in a mole is called Avogadro's number after the Italian chemist and physicist Amedeo Avogadro (1776-1856). Careful measurement determines Avogadro's number to be approximately 602.214 199 x 1021 entities per mole. In the American system of naming big numbers, that's 602 sextillion 214 quintillion 199 quadrillion, give or take about 50 quadrillion. momme a traditional Japanese weight unit corresponding to the Chinese mace (see above). Jewelers continue to use the momme to measure the weight of cultured pearls; for this purpose it equals exactly 3.75 grams or 0.132 ounce. month (mo or mon) [1] a unit of time marked by the revolution of the Moon around the Earth. In many traditional societies the appearance of the first tiny crescent moon after the New Moon signaled the start of the month. This start of the month, based on the first appearance of the Moon, is still proclaimed from mosques in Islamic countries. Thus the lunar month is defined as the average interval between two successive moments of New Moon. Astronomers call this period the synodic month. Its length is 29.530 59 days. month (mo or mon) [2] a civil unit of time equal to approximately 1/12 year, but varying from 28 to 31 days. The Sun and the Moon are our traditional time keepers, but they are badly out of step with each other. A solar year equals approximately 12.368 lunar months. The large fraction in this number makes it difficult to design a calendar with a whole number of months in each year. There are at least three solutions to the problem: [i] Use leap months. In the traditional Chinese and Jewish calendars most years have 12 months, but some have a 13th month. In these luni-solar calendars the length of the year varies from 354 to 384 days. [ii] Define 12 synodic lunar months to be a year and don't worry about the length of the year. This is the solution of the Islamic calendar. Since the Islamic year has only 354 or 355 days, its length does not match the cycle of the seasons. [iii] Observe the solar year and let the months be 12 arbitrary periods; don't worry about the Moon. This is the solution adopted by the Romans, who established the civil calendar we use today. In this calendar, all months have 30 or 31 days except the second month, February. February has 28 days in ordinary years and 29 in leap years. See also year [2]. morgan (M) a unit of genetic separation used in genetics and biotechnology. If two locations on a chromosome have a 100% probability of being separated during recombination in a single generation, then the distance between those locations is one morgan. In practice, measurements are made in centimorgans. morgen a traditional unit of land area in Northern Europe. "Morgen" means "morning," and most likely the unit arose as the area a yoke of oxen could plow in one morning. The Dutch morgen, also used in Dutch colonies including old New York, equals about 2.10 acres or 0.850 hectare. In South Africa, this unit was defined to equal 10 246 square yards, which is 2.1169 acres or 0.8567 hectare. In Scandinavia and northern Germany, the morgen is a smaller unit equal to about 0.63 acre or 0.25 hectare. mu or mou a traditional unit of land area in China. The traditional mu is about 675 square meters or 800 square yards. However, the colonial customs authorities used a larger mu equal to 8273.75 square feet, 919.3 square yards, or 768.65 square meters. In modern China, the mu is often reckoned to be exactly 1/15 hectare, which is 666.667 square meters or 797.327 square yards. mud a traditional Dutch unit of volume for grains and other dry commodities. Originally varying from market to market, the unit was declared equal to the hectoliter (about 3.5315 cubic feet or 2.838 U.S. bushels) when the metric system was introduced in the Netherlands. With this definition it is still in use. mug an informal contraction of the metric slug. See TME. mutchkin a traditional Scottish unit of liquid volume. The mutchkin is about 15 British fluid ounces, which is about 426 milliliters or almost exactly 0.9 U.S. pint. mwe abbreviation for meter of water equivalent, a unit used in nuclear physics to describe the shielding around a reactor, accelerator, or detector. 1 mwe of any material (such as rock, gravel, etc.) is a thickness of that material providing shielding equivalent to one meter of water. MWe, MWt symbols used in the electric power industry to describe the size of generating plants. MWe is the symbol for the actual output of a generating station in megawatts of electricity; MWt is used for the heat energy, or thermal output, required to operate the generators. Thermal output is typically about three times the electric output. myria- (my-) a metric prefix meaning 10 000. This prefix was part of the original metric system of 1795 and was used throughout the nineteenth century, but it is now considered obsolete. The ancient Greek word myrios means countless, without number. This was modified by later Greeks to form a word myrioi meaning ten thousand. Our word myriad, meaning an indefinitely large number, originally meant the number 10 000. myriagram (myg) a metric unit of mass equal to 10 000 grams or 10 kilograms (about 22.046 pounds). Although it is considered obsolete now, the myriagram was a useful unit comparable to the English quarter or Spanish arroba. N nail an old English unit of length equal to 1/20 ell. Like the ell, the nail was used for measuring cloth; traditionally, it represented the length of the last two joints (including the fingernail) of the middle finger. The nail is equivalent to 1/16 yard, 1/4 span, 2.25 inches, or exactly 5.715 centimeters. nano- (n-) a metric prefix meaning 10-9, or one billionth. The prefix comes from the Greek word nanos, dwarf. nanometer (nm) a metric unit of distance equal to 10-9 meter. Introduced in 1951, the nanometer replaced the millimicron. One nanometer equals 0.001 micrometer or 10 angstroms. nautical mile (nmi, naut mi or NM) a unit of distance used primarily at sea. The nautical mile is defined to be the average distance on the Earth's surface represented by one minute of latitude. This may seem odd to landlubbers, but it makes good sense at sea, where there are no mile markers but latitude can be measured. Because the Earth is not a perfect sphere, it is not easy to measure the length of the nautical mile in terms of the statute mile used on land. For many years the British set the nautical mile at 6080 feet, exactly 800 feet longer than a statute mile; this unit was called the Admiralty mile. In 1929 an international conference in Monaco redefined the nautical mile to be exactly 1852 meters or 6076.115 49 feet, a distance known as the international nautical mile. The international nautical mile equals about 1.1508 statute miles. There are exactly 60 nautical miles in a degree [2], and usually 3 nautical miles in a league. nebuchadnezzar a huge wine bottle holding about 15 liters, 20 times the volume of a regular bottle. neper (Np) a unit expressing the ratio of two numbers as a natural logarithm. Quantities differ by 1 neper if one is e2 = 7.389056 times the other. In English-speaking countries, the decibel is used similarly: Two quantities differ by 10 decibels if one is 10 times the other, or by 1 decibel if one is 100.1 = 1.258925 times the other. One neper is equal to about 8.685 890 decibels. The neper is accepted for use with SI units, and it may become an SI unit in the future. The unit recognizes (in French) the British mathematician John Napier (1550-1617), the inventor of the logarithm. nest an old English unit of quantity equal to 3. This unit was often, but not always, used for items which nest together, such as 3 bowls or 3 hampers. net ton another name for the U.S. ton of exactly 2000 pounds (907.185 kilograms); this unit is also called the short ton. new candle another name for the candela, a unit of light intensity. new style (NS) a notation used after dates to indicate that the date is stated in the Gregorian calendar (the calendar now in general use) rather than in the Julian calendar (see year [2]). The notation is used primarily for Gregorian dates between 15 October 1582, when the Gregorian calendar was adopted in Catholic Europe, and 14 September 1752, when it was adopted in Britain. See also old style. newton (N) the SI unit of force. A force of one newton will accelerate a mass of one kilogram at the rate of one meter per second per second. The corresponding unit in the CGS system is the dyne; there are 105 dynes in one newton. In traditional English terms, one newton is about 0.224 809 pounds of force. Nineteenth-century engineers created another English unit of force, the poundal; there are approximately 7.233 01 poundals in a newton. The newton is named for Isaac Newton (1642-1727), the British mathematician, physicist, and natural philosopher. He was the first person to understand clearly the relationship between force (F), mass (m), and acceleration (a), expressed by the formula F = ma. newton meter (N m) the SI unit of torque. Torque, the tendency of a force to cause a rotation, is the product of the force and the distance from the center of rotation to the point where the force is applied. Torque has the same units as work or energy, but it is a different physical concept. To stress the difference, scientists measure torque in newton meters rather than in joules, the SI unit of work. One newton meter is approximately 0.7376 pound foot. nibble a unit of information used in computer science. A nibble is 4 bits or 1/2 byte. The cuter spelling nybble, suggested by byte, is sometimes used. In different contexts, a group of 4 bits is sometimes called a quadbit or a hexit. nip an informal unit of liquid volume. The term "nip" usually means "a small amount," with no precise equivalent. In Britain, however, a nip of spirits is considered to be 1/6 gill (about 22.95 milliliters or 0.776 U.S. fluid ounce). nit (nt) an MKS unit of luminance, equal to one candela per square meter, or 10-4 stilb. Nm abbrevation for "normal metric", a symbol for the density of worsted yarns. Yarn is described as a/b Nm if it is a-ply (has a individual strands) and there are b kilometers of yarn per kilogram. An example might be 2/40 Nw, describing a 2-ply yarn having 40 kilometers per kilogram. If the yarn is single-ply, the a is often omitted, so a single-ply yarn is described as b Nm. noggin a traditional unit of liquid volume, used primarily in Ireland. Like the gill, the noggin is often taken to equal 1/4 pint; sometimes it is taken to be 1/2 pint. nook a traditional unit of land area in Scotland and northern England. A nook of land is 20 acres or about 8.094 hectares. normal (N) a term used in chemistry to describe a solution having a concentration of 1 gram equivalent per liter. The normal concentration of an ion is effectively equal to the molar concentration divided by the valence (the number of free or missing electrons) of the ion. nox a unit of (low) illumination equal to 0.001 lux. This unit was used in Germany during World War II to describe permitted levels of lighting during air raids. "Nox" is the Latin word for "night." noy a unit of perceived noisiness introduced by the American acoustics engineer K. D. Kryter in 1959. An observer uses the unit to described the noise levels he or she experiences as multiples of a standard level. The standard level, one noy, is defined to be the noisiness of a random noise signal within the frequency band from 910 to 1090 hertz at a sound pressure level of 40 decibels. As you have already guessed, the unit's name is chosen so that its plural is pronounced "noise." A jet aircraft takeoff is rated at about 110 noys. NTU abbreviation for nephelometric turbitity unit, a unit used in measuring water quality. Turbidity is an optical property: the scattering and absorbtion of light by solids suspended in water. In other words, water is turbid if you can't see through it. An instrument called a nephelometer (from a Greek word meaning "cloudy") measures turbidity directly by comparing the amount of light transmitted straight through a water sample with the amount scattered at an angle of 90¡Æ to one side; the ratio determines the turbidity in NTU's. The instrument is calibrated using samples of a standard solution of a synthetic polymer called formazin. Drinking water should not have a turbidity above 1 NTU, although values up to 5 NTU are usually considered safe. Nw abbrevation for "normal worsted", a symbol for the density of worsted yarns. Worsted yarn is described as a/b Nw if it is a-ply (has a individual strands) and there are b 560-yard hanks of yarn per pound. An example might be 2/40 Nw, describing a 2-ply yarn having 40 hanks (22 400 yards) per pound. If the yarn is single-ply, the a is often omitted, so a single-ply worsted yarn is described as b Nw. nybble see nibble, above. O o' clock [1] a contraction of the phrase "of the clock," used in English after a statement of time. This phrase is a relatively recent invention; it has been traced to the early 1700's. Earlier, time was usually stated in hours and minutes, and this is still the case in most languages. Thus "10 o' clock" is "10 hours" in most of the world. o' clock [2] an informal angular measure which works by describing an angle in terms of the face of a standard (12-hour) clock. Each hour "o' clock" spans an angle of 30¡Æ, so "4 o' clock" means an angle of 120¡Æ measured from dead ahead or some other agreed-upon point of reference. octad a unit of quantity equal to 8. octane number a measure used to express the ability of gasoline to reduce engine knocking. Gasoline is a complex mixture of hydrocarbons: compounds containing hydrogen and carbon. Beginning chemistry students learn that "octane" is the name of a hydrocarbon whose molecules contain 8 carbon atoms and 18 hydrogen atoms, the 8 carbons being arranged in a long chain. The compound cars need to prevent knocking is not that octane but a different compound of 8 carbon atoms and 18 hydrogen atoms called iso-octane or, in the more precise language of chemical nomenclature, 2,2,4-trimethylpentane. (In an iso-octane molecule there are only 5 carbons in the chain. Carbons 6 and 7 are attached to the sides of the chain at the #2 position, and the last carbon is hooked onto the #4 position; chemists call this a branched hydrocarbon.) To determine the octane rating of gasoline, a sample of the gasoline is compared to a laboratory mixture of iso-octane and another hydrocarbon called heptane (heptane has 7 carbons and 16 hydrogens, with the 7 carbons in a chain). The mixture is adjusted until it has the same anti-knocking characteristics as the gasoline being tested. The octane rating is the percentage of iso-octane required in the laboratory mixture to produce this equality of knocking behavior. (In fact, it's even more complicated: there are two ways to do the test, and the number posted on the gas pump is the average of the iso-octane percentages obtained in the two methods.) There is a similar unit, cetane, used for rating diesel fuel. octant [1] a unit of angle measure equal to 1/8 circle, 45¡Æ, or pi/4 radians. octant [2] a unit of solid angle measure. One octant is 1/8 sphere, or pi/2 steradians, or about 5156.6 square degrees. octave a unit used in music to describe the ratio in frequency between notes. Two notes differ by one octave if the higher note has exactly twice the frequency of the lower one. octavillo a traditional Spanish unit of dry volume. The octavillo equals about 289 milliliters (a little more than a cup, in U.S. terminology). This is equivalent to about 17.64 cubic inches, 0.525 U.S. pint, or 0.5086 British imperial pint. Since octavillo means "eighth," one would expect the octavillo to be 1/8 of a gallon. However, the gallon unit is missing in recent Spanish terminology. There are 4 octavillos in a cuartillo, 16 in an almude, and 48 in a fanega. octet [1] a unit of quantity equal to 8. octet [2] another name for a byte, used primarily in telecommunications. The name refers to the definition of a byte as 8 bits. Oechsle see degree Oechsle. oersted (Oe) the CGS unit of magnetic field strength. The oersted is defined to be the field strength in a vacuum at a distance 1 centimeter from a unit magnetic pole. A field of one oersted generates a magnetomotive force of 1 gilbert per centimeter of conductor. There is no named MKS unit of field intensity, but the oersted is equivalent in MKS units to 79.577 472 ampere-turns per meter. The unit honors Hans Christian ¨ªrsted (1777-1851), a Danish physicist, who discovered electromagnetism in 1820. ohm [1] the SI unit of electric resistance. If a conductor is connected between two locations having different electric potential, then a current will flow through the conductor. The amount of the current depends on the potential difference, but it also depends on the extent to which the conductor resists the flow of current. This property of a circuit is called electric resistance, and it is measured in ohms. One ohm is the resistance which requires a potential difference of one volt per ampere of current. This is a small resistance; in practical circuits resistance is often measured in megohms, millions of ohms. The unit honors the German physicist Georg Simon Ohm (1787-1854). The capital Greek letter omega is used as the symbol for the ohm, since "O" would be easily misinterpreted as a zero. ohm [2] a traditional German unit of liquid volume, no longer in use. The ohm was generally equal to 1/6 fuder or roughly 150 liters (40 U.S. gallons). The unit was often spelled aume in English. ohm meter a unit of resistivity, measuring the extent to which a substance offers resistance to passage of an electric current. The resistivity of a conductor in ohm meters is defined to be its resistance (in ohms) multiplied by its cross-sectional area (in square meters) divided by its length (in meters). oitavo a traditional Portuguese unit of liquid volume comparable to 1/2 gallon in the U.S. system. The oitavo equals 1/32 fanega or about 1.73 liters (0.46 U.S. liquid gallon or 0.38 British imperial gallon). oka or oke a traditional unit of weight in Turkey and the eastern Mediterranean. The oka is approximately 2.8 pounds or 1.25 kilograms; its size varied somewhat over the expanse of the area formerly part of the Turkish empire. The oka is also used sometimes as a unit of liquid volume equal to a little more than a liter, representing the volume (roughly 1.25 liters) occupied by an oka (weight) of water or wine. old style (OS) a notation used after a date to indicate that the date is stated in the Julian calendar (see year [2]). The Julian calendar was replaced by the Gregorian calendar in 1582 in predominantly Catholic countries of Europe. It remained in use in Britain, a Protestant country, until 1752, and in Russia, an Orthodox country, until 1918. In addition, the first day of the new year in Britain, until 1752, was 25 March instead of 1 January. Thus George Washington, the first U.S. president, was born on 11 February 1731 OS, or 22 February 1732 in the Gregorian calendar. olympiad a unit of time equal to four years. In ancient Greece, the olympiad referred to the four-year interval between successive Olympic Games. The first Greek olympiad was the period 776-773 BC. This usage was revived when the modern Olympics began, in 1896. The period 1997-2000, called the 26th Olympiad of the modern era, is also the 694th olympiad by the Greek reckoning. omer a ancient unit of volume for grains and dry commodities, used in the Bible. The omer was equal to 0.1 ephah; this is believed to equal about 4.032 liters, 246.05 cubic inches, 0.9154 U.S.dry gallon, or 0.8869 British imperial gallon. onca, once, oncia, onza traditional names for the ounce unit in Romance languages. The Portuguese onca and Spanish onza equal 1/16 libra or about 28.69 grams (1.012 ounce); the French once equals 1/16 livre or about 30.59 grams (1.079 ounce). The Italian oncia or onza is no longer used, but traditionally it equaled 1/12 libra or about 27.3 grams (0.96 ounce). ons a Dutch unit of weight or mass, now considered to be a metric unit equal to the hectogram (100 grams, or about 3.5274 ounces). open window unit (owu) the original name of the unit of sound absorption now called the sabin. order of magnitude a logarithmic unit used to compare the sizes of quantities. Two quantities differ by one order of magnitude if one is 10 times the other, by two orders of magnitude if one is 10¡¤10 = 100 times the other, and so on. Thus a difference of n orders of magnitude means the larger quantity is 10n times the smaller one. osmole (Osm) a unit of osmotic pressure used in physical chemistry, cell biology, and medicine. If chemical solutions are separated by a semipermeable membrane (a membrane which resists the passage of dissolved substances but permits the passage of the solvent, usually water), then the solvent will tend diffuse across the membrane to equalize the concentrations. This process is called osmosis. Solutions with higher concentrations of dissolved substances are said to have higher osmotic pressure than solutions having lower concentrations; thus the solvent moves from an area of low osmotic pressure to an area of higher osmotic pressure. One osmole is the osmotic pressure of a one molar solution (that is, a solution with a concentration of one mole per liter of solvent) of a substance which does not dissociate, such as sugar (glucose) in water. Osmotic pressure depends on the total number of dissolved particles, so for a substance which dissociates into two ions, such as ordinary salt (sodium chloride), a one molar solution has an osmotic pressure of 2 osmoles. In practice, most measurements are in milliosmoles (mOsm). Typical values range from 20 mOsm for fresh water through 290 mOsm for typical human blood plasma to 1010 mOsm for salt water from the open ocean. ounce (oz) [1] a traditional unit of weight. The avoirdupois ounce, the unit commonly used in the United States, is 1/16 pound or about 28.3495 grams. The avoirdupois ounce also equals about 0.911 457 troy ounce. The word ounce is from the Latin uncia, meaning a 1/12 part, because the Roman pound was divided into 12 ounces. The word "inch," meaning 1/12 foot, has the same root. The symbol oz is from the old Italian word onza (now spelled oncia) for an ounce. ounce (oz) [2] a second traditional unit of mass or weight. The troy ounce, traditionally used in pharmacy and jewelry, is 1/12 of a troy pound, or about 31.1035 grams. Thus the troy ounce equals 1.09714 avoirdupois ounces. This unit continues to be used as a conventional measure for gold; in particular, the price of gold (as quoted in financial markets) is the price per troy ounce. The troy ounce is sometimes abbreviated oz t to distinguish it from the more common avoirdupois ounce. ounce (oz) [3] a traditional unit of liquid volume, also called the fluid ounce (fl oz). ounce of force (ozf or oz) a traditional unit of force, equal to the force experienced at the earth's surface by a mass of one ounce. One ounce of force equals 1/16 pound of force or about 0.278 014 newtons. P pace a traditional unit of distance, considered equal to the length of a person's "full" pace, that is, the distance between two successive falls of the same foot. Thus one pace equals two steps. The Romans counted 1000 paces in a mile, with each pace being a little over 58 inches (or about 148 centimeters). In more recent times, the pace has usually been defined as exactly 5 feet (or 152.4 centimeters); this unit is called the great pace or geometrical pace. In the military, however, the term "pace" is often used as an alternate name for the step; see military pace. pack (pk) a commercial unit specifying the number of items per package. In retail trade, packages containing 4 items (for example) are often described as "4 pack" or "4pk." The symbol pk is also used for the peck (see below). packen a traditional Russian unit of weight equal to 1200 funte, 30 pudi (see pud below), 1083 pounds, or 491.4 kilograms. pair (pr) a unit of quantity equal to 2. The word is from the Latin paria, meaning (two) equals. palm [1] a traditional unit of distance equal to the width of a person's palm. In the ancient world, the palm was considered equal to 4 digits or 1/6 cubit, which would be about 3 inches or 7.5 centimeters. This unit was used very commonly in medieval and early modern Britain. palm [2] In the U.S., perhaps due to familiarity with the Spanish palmo (see next entry), the palm is more often an informal unit equal to 9 inches (22.86 centimeters); this makes the U.S. palm an alternate name for the span. palmo a traditional unit of distance in Spain and Portugal. The traditional Spanish palmo equals 9 pulgadas (see below) or 1/4 vara: this is about 20.9 centimeters in Spain and a little more than that in Spanish Latin America. In Texas, 1/4 vara comes to 8 1/3 inches (21.17 centimeters). Under the metric system in Spain, the palmo is an informal unit equal to 20 centimeters. The Portuguese palmo equals 0.1 braca or about 22.0 centimeters (8.66 inches). These units are based on the length of a person's hand, from the wrist to the tip of the middle finger. parasang a historic unit of distance comparable to the European league. The unit originated in Persia but was used throughout the ancient Middle East and Mediterranean. It was equal to roughly 3.5-4.0 miles or about 6 kilometers. Paris foot an English name for the French royal foot (pied de roi, see below). parsec (pc) a non-metric unit of distance used in astronomy. As the Earth makes its orbit around the Sun, nearby stars appear to shift their positions in relation to the background of distant stars. The shift, called the parallax of the star, is very small, less than one arcsecond even for the nearest stars. If it can be measured, however, then simple trigonometry can be used to find the distance from the Earth to the star. One parsec is the distance at which a star appears to shift its position by one arcsecond over the course the time (about 3 months) in which the Earth moves a distance of one astronomical unit (au) in the direction perpendicular to the direction to the star. Using this unit makes it easy to compute distances: the distance to a star, in parsecs, is simply one divided by the amount of the parallax. If the parallax is 0.01 seconds, the distance is 100 parsecs. The length of a parsec, divided by one astronomical unit (the radius of the Earth's orbit) is the trigonometric function of the parallax called the cotangent; from this relation we can compute that one parsec equals 206 264.8 au. This is the same as 3.261 631 light years, 30.856 78 x 1012 kilometers, or about 19 173 510 000 000 miles. particle flux unit (pfu) a unit used to measure the rate at which energetic particles, such as protons, are received by spacecraft. These flux rates are a major component of the "space weather," the environment in which satellites and other spacecraft operate. One pfu is a rate of one particle per square centimeter of detector area per steradian of solid angle scanned per second of time. (A steradian is about 7.96% of a sphere.) In SI units, 1 pfu = 10-4 m-2sr-1s-1. part per billion (ppb) a unit of proportion, equal to 10-9. part per million (ppm) a unit of proportion, equal to 10-6. part per trillion (ppt) a unit of proportion, equal to 10-12. The symbol ppt has also been used for part per thousand, that is, 10-3, but that unit should be called per mill (see below). pascal (Pa) the SI unit of pressure. The pascal is the standard pressure unit in the MKS metric system, equal to one newton per square meter or one "kilogram per meter per second per second." Sounds impressive, but in traditional English terms a pascal is only 0.000 145 pounds per square inch (0.020 885 lbf/ft2 or 0.007 50 mmHg). Thus pressure is more commonly measured in kilopascals (kPa), with 1 kPa = 0.145 lbf/in2. Air pressure is measured in hectopascals (hPa), with 1 hPa = 1 millibar. The unit is named for Blaise Pascal (1623-1662), French philosopher and mathematician, who was the first person to use a barometer to measure differences in altitude. pe the traditional Portuguese foot, equal to 12 polegadas or about 33.324 centimeters (13.12 inches). pebi- (Pi-) a binary prefix meaning 250 = 1 125 899 906 842 624. This prefix, adopted by the International Electrotechnical Commission in 1998, replaces peta- for binary applications in computer science. The prefix is a contraction of "petabinary." peck (pk) a traditional unit of volume, formerly used for both liquids and solids but now used mostly for dry commodities such as grains, berries, and fruits. A peck is 2 gallons, 8 quarts, or 1/4 bushel. In the U. S. customary system, a peck holds 537.605 cubic inches, or approximately 8.8098 liters. In the British imperial system, a peck is a little larger, holding 554.84 cubic inches, or approximately 9.0923 liters. The word "peck", originally spelled "pek", comes from the name of a similar French unit. -penny an ending added to a number to indicate the size of a nail, as in "sixpenny nail" or "tenpenny nail." It's not clear exactly how this terminology began, although the usual guess is that tenpenny nails originally cost ten pence per hundred. There is, very roughly, a linear relation between the size designations and length: an n-penny nail is roughly (1/2) + (1/4)n inches long. This makes the tenpenny nail about 3 inches long, the eightpenny about 2.5 inches, and so on. pennyweight (dwt or pwt) a unit of weight in the traditional troy system (see pound [2]), equal to 24 grains or 1/20 troy ounce. One pennyweight is approximately 1.5552 gram. The d in the traditional symbol dwt is from the Latin word denarius for the small coin which was the Roman equivalent of a penny. (The letter d is also the traditional symbol for the penny in the English monetary system.) pentad [1] a unit of quantity equal to 5. pentad [2] a unit of time equal to 5 days. This unit is common in meteorology, where forecasts are frequently made for periods of 5 days at a time. per annum (PA) a traditional unit of frequency equal to once a year. percent (% or pct) a unit of proportion, equal to 0.01. The word is Latin, meaning "by the hundred." The symbol % can be placed after any number; mathematically, its effect is an immediate division by 100. percentile a unit used in statistics to describe a portion of the individuals or events being studied. Suppose the data are arranged by numerical scores, from highest to lowest. A score belongs to the 78th percentile, for example, if it is greater than 78% of the scores but it is not greater than 79% of the scores. This procedure divides the scores into 100 percentiles, numbered 0th through 99th. A group of 10 consecutive percentiles is called a decile; 25 consecutive percentiles are called a quartile. perch [1] an alternate name for the rod (16.5 feet or 5.0292 meters), introduced in the twelfth century by the Norman conquerors of England. The word perch (perche in French: see below) comes from the Latin pertica (pole). The Romans also had a distance unit called the pertica, but it was shorter: 10 Roman feet (9.71 English feet or 2.96 meters). perch [2] a unit of area equal to one square perch [1]. A perch of area covers exactly 272.25 square feet or about 25.292 85 square meters. perch [3] a traditional unit of volume for stone and other masonry. A perch of masonry is the volume of a stone wall one perch (16.5 feet) long, 18 inches high, and 12 inches thick. This is equivalent to exactly 24.75 cubic feet, 0.916 667 cubic yard, or about 0.700 842 cubic meter. perch [4] a traditional unit of distance in Ireland standardized at 21 English feet (6.4008 meters) or 14/11 English perch or rod. Since the Irish perch was 27.27% longer than the English, the Irish chain, furlong, and mile were longer in the same proportion.. perche or perch [1] a traditional unit of distance in French North America. The perche equals 18 pieds (see below) or 3 toises. By legal definition in Canada this equals 19.1835 English feet or 5.847 13 meters. perche or perch [2] a traditional unit of area in French North America, equal to one square perche [1]. A perche of area is therefore equal to 0.01 arpent, about 368.007 square feet (40.8896 square yards) or 155.330 square meters. per diem (PD) a traditional unit of frequency equal to once a day. perfect ream a unit of quantity for paper. An ordinary ream is 480 or 500 sheets; a perfect ream is 516 sheets. The additional amount is to allow for sheets which may be spoiled in shipment. per mill, per mil a unit of proportion, equal to 0.001. Unlike percent, per mill is written as two words. Its symbol, not available to most web browsers, is like the percent symbol but with two zeroes in the denominator (roughly, 0/00). The spelling "per mill" seems to be more common in the U.S., "per mil" being more common in Britain. person hour a gender-nonspecific version of man hour, a unit of labor equal to the work of one person for one hour. peta- (P-) a metric prefix denoting 1015 (one quadrillion). One parsec, for example, equals 30.857 petameters. The prefix is drawn from a Greek root which means "spreading out"; our word petal has the same root. pferdestarke (ps) German word for horsepower, meaning the metric horsepower. The symbol ps is used for horsepower in both the Japanese and German automotive industies. pfu symbol for the particle flux unit (see above). pfund (pfd) a traditional German weight unit corresponding to the English pound (see below). The pfund is equal to 16 unzen or 32 lote. Traditionally the pfund varied in size from market to market, and the various German states adopted different standards, ranging from something close to the English pound (454 grams) to the Viennese pfund at about 1.2 pounds (560 grams). When Germany was unified in the late nineteenth century, the pfund was redefined as a metric unit equal to exactly 500 grams (about 1.102 31 pound). The plural is pfunde. pH a logarithmic unit used to measure the acidity or alkalinity of a chemical solution. The properties of a liquid solution we call "acid" are caused by the presence of hydrogen ions (H+). The pH of a solution is a measure of the concentration of these hydrogen ions. Technically, the pH of a solution is defined to be the negative logarithm of the concentration, measured in moles per liter. This unit is inverted in the sense that lower pH readings correspond to greater acidity, and therefore more hydrogen ions. Lowering the pH by 1.0 means multiplying the ion concentration by a factor of 10. The pH scale runs from to 14. A neutral solution (neither acidic nor alkaline, like pure water) measures 7.0 pH. Numbers below 7 indicate increasing acidity, while numbers above 7 indicate increasing alkalinity. The pH scale was invented by S. P. L. Sorenson in 1909. phon a unit of sound loudness closely related to the decibel. Decibels are used for objective measurements, that is, they measure the actual pressure of the sound waves as recorded using a microphone. Phons are used for subjective measurements, that is, measurements made using the ears of a human listener. A sound has loudness p phons if it seems to the listener to be equal in loudness to the sound of a pure tone of frequency 1 kilohertz and strength p decibels . A measurement in phons will be similar to a measurement in decibels, but not identical, since the perceived loudness of a sound depends on the distribution of frequencies in the sound as well as the pressure of the sound waves. In the U.S., sound loudness is frequently measured in sones rather than phons: a sound of loudness s sones has loudness 10 log2 s + 40 phons. In English the unit is pronounced "fon" rather than "phone." phot (ph) the CGS unit of illumination, equal to one lumen per square centimeter or 10 000 lux. See also lambert. The name is usually pronounced to rhyme with "not" rather than "note." photon [1] a quantum of light energy, or, more generally, of any form of electromagnetic energy having a single wavelength, direction, and polarization. photon [2] a former name for the unit of retinal illuminance, now called the troland. pi a very famous mathematical unit. The circumference of a circle is equal to pi multiplied by the diameter, so pi is equal to the ratio between the circumference and diameter of a circle, any circle. It turns out that pi is an irrational number, which means that its decimal expansion is nonterminating and nonrepeating. To 25 significant digits, pi equals 3.141 592 653 589 793 238 462 643. The Swiss mathematician Leonard Euler (1707-1783) adopted the lower case Greek letter pi for this ratio in 1737; although he was not the first to use it, he popularized it through his many mathematical writings. Pi sometimes appears to be a unit of angle measure equal to pi radians or 180¡Æ. This is because mathematicians regard angle measurement as dimensionless (length divided by length) and therefore omit the unit "radians". pic see pik, below. pica (pi) a unit of length used by typographers and printers. One pica equals 12 points, 4.22 mm, or a bit less than 1/6 inch. "Pica type" is type that sets six lines to the inch. The origin of this word is not known. In the Catholic Church, a "pica" is a book of daily services, so it is possible that pica type got its name from being used to print picas. The unit is pronounced "pike-ah." pico- (p-) a metric prefix denoting 10-12 (one trillionth). The prefix, pronounced "peek-o" in English, is coined from the Italian piccolo, small. picocurie (pCi) a common unit of radioactivity, used to measure radioactivity occurring naturally in the environment. One picocurie equals 10-12 curie or 0.037 becquerel; this corresponds to one atomic disintegration about every 27 secondsa very low rate of activity. picul a unit of weight comparable to the European quintal, widely used in East Asia during the colonial period. The picul is equal to 100 catties, typically about 133.3 pounds or 60.5 kilograms. In recent years the picul has been used as a metric unit equal to 60 kilograms (132.28 pounds) in Thailand or 50 kilograms (110.23 pounds) in China. The unit is pronounced "pickle." pie the traditional foot of Italy and Spain. The Italian pie, no longer used, was about 29.8 centimeters. The Spanish pie equals 1/3 vara or 12 pulgadas (see below). The pie used in Spain is about 27.86 centimeters or 10.97 inches, but in Spanish Latin America the pie is generally longer. The Argentine length is 28.89 centimeters or 11.37 inches; in Texas, 1/3 vara is 11 1/9 inches or 28.22 centimeters. piece (pc) a unit of quantity, equal to 1. This unit, like count (ct) is used to indicate that a measurement represents an exact count of items. pied the traditional French foot. Pieds of various lengths were used in France, but the one best remembered now is the royal foot (pied de roi), called the Paris foot in English and the foot (French measure) in Canadian law. The pied de roi equals about 32.48 centimeters or 12.79 inches; the official Canadian definition is 12.789 inches (32.484 06 centimeters). pieze (pz) a metric unit of pressure, part of the "metre-tonne-second" system sometimes used by European engineers. The pieze is a pressure of one sthene per square meter, or 1000 newtons per square meter, or one kilopascal. This is not such a large pressure; one pieze equals 10 millibars or about 0.145 pound per square inch. The name of the unit comes from the Greek piezein, to press. The unit, spelled pieze in French, is pronounced "pee-ezz" in English. pik or pic a traditional unit of distance in the Eastern Mediterranean and Near East. The pik varied considerably but a typical value is about 28 inches (71 centimeters). This is an "arm" unit, like the Italian braccio and the Russian arshin. pin a traditional British unit of volume, used for beer. A pin is very different from a pint: it is equal to 1/8 barrel or 4.5 imperial gallons (20.457 liters). There are 2 pins in a firkin. pinch an informal unit of volume used in food recipes. In the U.S., the pinch is usually defined to equal 1/8 teaspoon, which is 1/48 fluid ounce or about 0.6 milliliter. pint (pt) [1] a traditional unit of volume equal to 1/2 quart. There are three different quarts in use in Britain and the United States, and hence there are three different pints: [i] the U. S. liquid pint, equal to exactly 28.875 cubic inches, 16 fluid ounces, or approximately 473.176 milliliters; [ii] the U. S. dry pint, equal to 33.600 cubic inches or approximately 550.611 milliliters; and [iii] the British imperial pint, equal to 34.678 cubic inches or approximately 568.261 milliliters. In Scotland, the pint is a much larger unit equal to 4 mutchkins, 103.404 cubic inches, 1.491 British imperial quart or 1.695 liter. The origin of the word pint is unclear. pint (pt) [2] a unit of volume used in Australian pubs. A pint of beer is generally 425 milliliters, or roughly 3/4 imperial pint. pipa a traditional Portuguese unit of liquid volume, originally very similar in size to the English pipe (see next entry). The pipa has become a metric unit equal to exactly 500 liters, which is 0.5 cubic meter, 132.085 U.S. gallons, or 109.996 British imperial gallons. pipe like the butt, the pipe is a traditional unit of liquid volume generally equal to 2 hogsheads. In the U.S., this means a pipe equals 126 U.S.gallons, about 16.844 cubic feet or 476.96 liters. In Britain, things are more complicated, because traditional British hogsheads were of different sizes depending on what they contained. The British pipe was usually used as a wine measure, but even different types of wine had different size pipes. planck an MKS unit of "action" (energy expended over time) or of angular momentum. The planck is equal to 1 joule second or about 0.7375 foot pound second. The unit honors the German physicist Max Planck (1858-1947), the originator of quantum theory. PM or pm abbreviation for the Latin post meridiem, "after noon," used after a time to indicate that the time occurs after 12:00 noon. Thus 4:30 pm is the same as 16:30. The notations "AM" and "PM" are used extensively in the United States, where time is usually not stated on a 24-hour basis. PMPO abbreviation for "peak music power output," which is often claimed by electronics manufacturers as a unit measuring the effective power output of amplifiers, stereo systems, etc. Buyer beware! There is no industry standard for this "unit," so it is impossible to determine just what it means. point (pt) [1] a unit of angle measure equal to 1/32 of a circle. Compasses are read by suspending the compass needle over a compass card traditionally inscribed with a 32-point star. Each point of the star represents a named direction; for example, the first five points from north towards east are labeled North, North by East, North Northeast, Northeast by North, and Northeast. The difference between two directions can be expressed as a certain number of these compass points. One point equals 11¡Æ15' of arc or pi/16 radians. point (pt) [2] a unit of length used by typographers and printers. When printing was done from hand-set metal type, one point represented the smallest element of type that could be handled, roughly 1/64 inch. Eventually, the point was standardized in Britain and America as exactly 0.013 837 inch, which is about 0.35 mm (351.46 micrometers) and a little bit less than 1/72 inch. In continental Europe, typographers traditionally used a slightly larger point of 0.014 83 inch (roughly 1/67 inch), sometimes called a Didot point after the French typographer Firmin Didot (1764-1836). The distinction between the British and Didot points is rapidly disappearing, since most digital typesetting and page design software now define the point to be exactly 1/72 inch (0.013 888 9 inch or 0.352 777 8 millimeters). point (pt) [3] a unit of proportion, equal to 0.01 or 1%, often called a percentage point. point (pt) [4] a unit of mass used for precious stones such as diamonds. One point equals 0.01 carat, or exactly 2 milligrams. point (pt) [5] a unit of quantity equal to 1. This unit is used to express changes in an arbitrary score or index, such as the score in an athletic contest. In finance, a change of one point in the Dow Jones average or similar indices represents a change of 1.00 in the index. point (pt) [6] a unit used to represent the smallest significant change in an arbitrary ratio. This usage is common in sports. Most sports "averages" are actually ratios of successful performances divided by attempted performances; baseball's batting average is a good example. These ratios are computed to a fixed number of decimal places, usually three, and a point represents a change of 1 in the last decimal place. Thus the batting averages .314 and .302 are said to differ by 12 points. point (pt) [7] another name for a mil [1], a unit of distance equal to 0.001 inch (25.4 micrometers). Points are used with this meaning to measure the thickness of card stock in the paper industry. point (pt) [8] a measure of the specific gravity of a liquid, typically used in brewing and winemaking. Specific gravity is the mass of a sample of the liquid divided by the mass of an equal volume of pure water. It is a dimensionless (unit-less) number, typically a little larger than 1. Each "point" represents an increase of 0.001 above 1. For example, a liquid of specific gravity 1.048 is described as 48 point. poise (P) a CGS unit of dynamic viscosity. Viscosity is a frictional property (actually, several related properties) of liquids and gases: due to friction between molecules, the liquid or gas resists flowing to a greater or lesser extent. Dynamic viscosity is measured by stating the force needed to move a standard area of one layer of the liquid or gas with respect to another layer, the two layers being parallel and separated by a standard distance filled with the same liquid or gas. If a force of one dyne is needed to move one square centimeter of the liquid or gas relative to a second layer one centimeter away at a speed of one centimeter per second, then the viscosity is one poise. The unit is named for the French physician Jean Louis Marie Poiseuille (1799-1869). Generally speaking, liquids have viscosities measured in centipoises and gases have viscosities measured in micropoises. The SI recognizes no named unit for dynamic viscosity; in SI units, one poise equals 0.1 pascal second (Pa¡¤s). The poise is also equivalent to 0.067 196 9 reyn. pole [1] another name for a rod. pole [2] see unit magnetic pole. polegada the Portuguese "inch" unit, equal to 1/12 pe, 2.777 centimeters, or 1.093 inches. pond [1] the Dutch pound, now reinterpreted as a metric unit equal to exactly 500 grams (1.1023 pounds), like the German pfund. pond (p) [2] a unit of force formerly used in metric countries. A pond is the gravitational force on a mass of one gram; thus it is equal to 980.665 dynes or 0.002 204 622 6 pounds of force. The kilopond was used more often than the pond. pony a small glass for liquor. In the U.S., a pony generally holds exactly 1 (U.S.) fluid ounce or about 29.6 milliliters. pood see pud. pot [1] a traditional unit of volume in many countries of Europe, roughly comparable to the liter or to the English quart. In Switzerland, the pot is now a metric unit equal to 1.5 liters. In Belgium, the pot is interpreted as 1.5 liters for dry quantities, but only 0.5 liters for liquids. The traditional pot is equal to 0.967 liters in Denmark and to 0.965 liters in Norway. pot [2] a unit of volume used in Australian pubs. A pot of beer is 285 milliliters in Queensland and Victoria, 425 milliliters in Western Australia. pottle a traditional unit of volume equal to 1/2 gallon. The unit's name is from the French potel, a type of bottle. pouce the French "inch" unit, equal to 1/12 pied (see above). Based on the pied de roi, the pouce equals about 1.066 inches or 2.707 centimeters. The word pouce means "thumb" in French. poumar an acronym for one pound per million yards, a unit of yarn density formerly used in the U.S. textile industry. One poumar equals about 0.496 055 tex. pound (lb or #) [1] a traditional unit of mass or weight. The Romans used a pound (the libra pondo, "pound of weight") divided into 12 ounces. All the countries of western Europe used similar units, divided into 12 or 16 ounces, until the advent of the metric system. 12-ounce pounds were common in Italy and southern France, but in Spain and northern Europe 16-ounce pounds became the norm. The word libra is used for this unit in Italy, Spain, and Portugal; in France it is called the livre. Further north, the Latin word pondo ("weight") is the origin of the names of the English pound, Dutch pond, Danish pund, German pfund, and Russian funt. In England, two different "pound" units became standard. The unit now in general use in the United States is the avoirdupois pound, so-called from a French phrase avoir du poids, literally "goods of weight," indicating simply that the goods were being sold by weight rather than by volume or by the piece. The avoirdupois pound is divided into 16 ounces. By international agreement, one avoirdupois pound is equal to exactly 453.592 37 grams; this is about 1.215 28 troy pounds. The avoirdupois pound is sometimes abbreviated lb. av. or lb. ap. to distinguish it from the less common troy pound. pound (lb t or lb or #) [2] a second traditional unit of mass or weight. The troy pound, named for the French market town of Troyes, was the unit used in England by apothecaries and jewelers. The troy pound is divided into 12 ounces, like the Roman pound. One troy pound is 373.242 grams, or about 0.822 858 avoirdupois pounds (13.165 72 avoirdupois ounces). The troy and avoirdupois pounds are connected by the grain: there are 5760 grains in a troy pound and 7000 grains in an avoirdupois pound. The troy pound should be abbreviated lb. t. to distinguish it from the more common avoirdupois pound. pound (lbf or lb) [3] a traditional unit of force; see pound force, below. pound (lb) [4] a traditional unit measuring the weight of paper; see pound weight, below. poundal (pdl or pl) an English unit of force used in engineering. Since traditional measuring systems, including the English system, did not distinguish between force and mass units, nineteenth century engineers invented the poundal to provide a unit which clearly measures force rather than mass. One poundal is the force which accelerates a mass of one pound at the rate of one foot per second. Since the acceleration of gravity averages about 32.174 ft/sec2 at the Earth's surface, one poundal is about 1/32.174 = 0.031 081 pound of force. One poundal is also equal to approximately 0.138 26 newton, or 13 826 dynes. The newton, the SI unit, is now the preferred unit of force in engineering and technical work. pound foot (lbf ft or lb ft) a traditional unit of torque. Torque is the tendency of a force to cause a rotation; it is the product of the force and the distance from the center of rotation to the point where the force is applied. Thus it can be measured in pounds of force times feet of distance. One pound foot is equal to approximately 1.355 82 newton meter (N m) in SI units. Torque has the same units as work or energy, but it is a different physical concept; to stress the difference, scientists and engineers measure torque in pound feet (or newton meters) and work or energy in foot pounds (or joules). pound force (lbf or lb) a traditional unit of force. Traditional measuring systems did not distinguish between force and mass units. A force of one pound is simply the gravitational force experienced at the Earth's surface by a mass of one pound. To compute this force, we multiply the mass by the acceleration of gravity, following Newton's law F = ma. Since one pound of mass is 0.453 592 kilograms and the acceleration of gravity averages 9.806 65 meters per second per second at the surface of the Earth, one pound force equals the product of these two numbers, 4.448 22 newtons. The symbol lbf should be used for the pound force to distinguish it from the pound of mass. pound mole (lbmol) a unit of amount of substance. One pound mole of a chemical compound is the same number of pounds as the molecular weight of a molecule of that compound measured in atomic mass units. Thus the pound mole is equal to exactly 453.592 37 moles. pound per square foot (lbf/ft2 or psf) a traditional unit of pressure. 1 psf equals about 47.880 pascals (Pa), 0.478 80 millibars (mb), or 0.192 79 inches of water (in WC). pound per square inch (lbf/in2 or psi) a traditional unit of pressure. 1 psi equals 144 pounds per square foot (psf), 6.894 75 kilopascals (kPa), 68.9475 millibars (mb), 2.036 inches of mercury (inHg), or 27.7612 inches of water (inWC). See below for related notations such as "psig." pound weight (lb wt or lb) a traditional unit measuring the weight or thickness of paper. Paper is described as, say, 24 pound weight if one ream (500 sheets) cut 25 by 38 inches (63.50 by 96.52 centimeters) has a mass of 24 pounds. power (x) [1] a unit expressing the magnifying power of an optical system. The power is defined to be the angular diameter of the image formed by the system divided by the angular diameter of the original object being observed. In simple telescopes this is equal to the focal length of the primary objective (the big lens or mirror) divided by the focal length of the eyepiece lens. For binoculars, the power is customarily followed by the diameter of the objective lenses, in millimeters, so "8x40" indicates binoculars with a magnifying power of 8 and lenses of diameter 40 mm. power (x) [2] similarly, power is used in a variety of other situations to indicate that a measurement is a multiple of some standard quantity. For example, in computer technology, a 16x CD-ROM drive spins a disk 16 times faster than a "standard" speed drive. ppi abbreviation for pixels per inch. A pixel is a single "picture element", so ppi measures the resolution, or fineness, of an image. The unit is identical to dots per inch (dpi). proof (prf) a traditional unit of proportion used for measuring the strength of distilled liquors, including medicinal solutions of alcohol as well as alcoholic beverages such as whiskey. The proof rating of a liquor is the alcohol content of the liquid expressed as a percentage of the alcohol content of a standard mixture, called the proof liquor. In the United States, the proof liquor is legally defined so to contain exactly 50% alcohol measured by volume. As a result, the U. S. proof rating is equal to exactly twice the percentage of alcohol present, measured by volume. Thus "86 proof" means 43% alcohol. In Britain proof ratings are no longer used, but the former proof liquor contained 57.27% alcohol by volume. ps see pferdestarke above. psi, psia, psid, psig traditional symbols for pressure units used in hydraulics and plumbing. psi is a symbol for pound per square inch (see above). psig is a symbol for pound per square inch guage; this means that the pressure has been read from a guage which actually measures the difference between the pressure of the fluid and the pressure of the atmosphere. psia means pound per square inch absolute, which is the total pressure including the pressure of the atmosphere. psid, pound per square inch differential, is a symbol for a difference between two pressures, neither of which is atmospheric pressure. Corresponding symbols for pound per square foot (psf, etc.) are also used. pu A unit of distance used during the colonial era in China. The pu equals 5 ch'ih, 5.875 feet, or 1.7907 meters. pud or pood a traditional unit of weight in Russia. The pud equals 40 funte or 1/30 packen; this is about 16.381 kilograms or 36.11 pounds. The plural is pudi. puff (pF) an informal name for the picofarad , a unit of electric capacitance. pulgada the traditional Spanish inch, equal to 1/12 pie (see above). The pulgada varies from about 23.2 to 24.1 millimeters (0.913 to 0.949 inch). puncheon a traditional unit of liquid volume. The puncheon is often reckoned as equal to 70 gallons. In the U. S. system that would be about 9.358 cubic feet or 264.98 liters; in the British imperial system it would be about 11.238 cubic feet or 308.34 liters. There are other versions of the unit; in one version a puncheon of wine equals 84 wine (or U.S.) gallons (roughly 308 liters); in another, a puncheon of beer equals 72 gallons (roughly 272.5 liters). pund the Scandinavian pound, now reinterpreted as a metric unit equal to 500 grams (1.1023 pounds), like the German pfund. Q QD abbreviation for the Latin quaque die, once a day, a unit of frequency traditionally used in medical prescriptions. QH abbreviation for the Latin quaque hora, once an hour, a unit of frequency traditionally used in medical prescriptions. qian a traditional Chinese weight unit. In modern China the qian is equal to 0.1 liang, or exactly 5 grams (0.1764 ounce). QID abbreviation for the Latin quater in die, four times a day, a unit of frequency traditionally used in medical prescriptions. quad a unit of energy equal to 1015 (one quadrillion) Btu or about 1.055 exajoules (EJ) or 293.07 terawatt hours (TWh). quadbit a unit of information equal to 4 bits or 1/2 byte. This unit is used in telecommunications, where data is frequently transmitted in quadbits. In other contexts, the same unit is called a nibble or a hexit. quadrant (quad) a unit of angle measure, equal to 1/4 circle, pi/2 radians, 90¡Æ, or 100 grads. quadrat- (q-) a German prefix meaning "square." For example, the square kilometer is the quadratkilometer (qkm or km2) in German. quadrennium a traditional unit of time equal to four years. quantum a unit of energy used in physics. At the small scales studied in particle physics, energy occurs only is discrete packets or units called quanta. The amount of energy in a quantum depends on the frequency of the radiation carrying the energy; it is equal to the frequency (in hertz) multiplied by Planck's constant, 6.626 069 x 10-34 joule second (J s). quart (qt) [1] a traditional unit of volume, so-called because it equals exactly 1/4 (one quarter) of a gallon. However, there are several possible gallons to consider: [i] in the U. S. customary measure system for liquid volumes (such as milk, for example), one quart is exactly 57.75 cubic inches, 32 fluid ounces, or approximately 0.946 36 liters; [ii] in the U. S. customary measure system for dry volumes (pecans or strawberries, for example), one quart is 67.201 cubic inches, or approximately 1.1012 liters; [iii] finally, in the British imperial system, used for both liquid and dry commodities, one quart is 69.354 cubic inches, 40 fluid ounces, or approximately 1.1365 liters. (Units in the British imperial volume system are about 20% larger than U. S. liquid units, and about 3.2% larger than U. S. dry units.) In all cases, the quart equals 2 pints. quart (qt) [2] a unit of volume, smaller than the standard quart [1], used for measuring wine. Wine bottles have often been called "quarts," although they were smaller than standard quarts. In the U.S., wine was often measured by the champagne quart, which contains only 26 U.S. fluid ounces instead of 32. This is equivalent to about 46.92 cubic inches or approximately 768.92 milliliters. In Britain, wine was sold by the reputed quart. Following the establishment of imperial measure, the reputed quart was fixed at 2/3 imperial quart, which is equivalent to exactly 26 2/3 fluid ounces, about 46.24 cubic inches, or 757.67 milliliters. However, these measures have largely disappeared in favor of the international wine bottle containing exactly 750 milliliters. quarter (qtr or Q) [1] a traditional unit of weight equal to 1/4 hundredweight. In Britain, one quarter equals 28 pounds (12.7006 kilograms); in the United States, one quarter equals 25 pounds (11.3398 kilograms). In the U.S. "quarter" is also used informally to mean 1/4 ton, or 500 pounds. quarter (qtr or Q) [2] a civil unit of time, equal to 3 months or 1/4 year. The quarter is widely used as a time unit in business and economics. Given the layout of the Julian/Gregorian calendar in civil use throughout the world (see year [2]), the quarter varies in length from 90 to 92 days depending on its starting date. quarter (qtr or Q) [3] a unit of angle measure equal to 1/4 circle; another name for a quadrant. quarter (qtr or Q) [4] a unit of angle measure, sometimes used at sea, equal to 1/4 of a compass point (see point [1]). In this use, a quarter equals 2¡Æ48'45" = 2.8125¡Æ or pi/64 radians. quarter (qtr or Q) [5] a traditional unit of volume used for measuring grain. A quarter of grain is 8 bushels (about 281 liters, based on the U.S. bushel, or 291 liters, based on the British imperial bushel), perhaps because this quantity of grain weighs roughly 1/4 ton. quarter (qtr or Q) [6] an informal unit of distance equal to 1/4 mile, 2 furlongs, or 402.336 meters. This unit is used in athletics and horse racing. quarter (qtr or Q) [7] an informal unit of time equal to 1/4 hour or 15 minutes. This unit occurs in informal expressions of time, such as "quarter after 10" for 10:15. quarter (qtr or Q) [8] a unit of relative time used in sports, equal to 1/4 the total playing time of a competition. A quarter is 15 minutes in American football, 12 minutes in professional basketball. quartern [1] an old English word for a quarter of anything, "quartern" has been used to represent 1/4 of various units. In the U.S., the term seems to have been used mostly as an alternate name for the gill, which is equal to 1/4 pint or about 118.3 milliliters. quartern [2] a traditional English unit of weight equal to 1/4 stone, 3.5 pounds, or about 1.5876 kilogram. See also quartern-loaf, below. quartern [3] a traditional unit of volume for dry goods equal to 1/4 peck or 2 quarts. This corresponds to 2.2731 liters in the British imperial system or 2.2025 liters in the U.S. system. The unit is much more common in Britain. quartern [4] a traditional English unit of quantity equal to 25, or 1/4 of 100. quartern-loaf a traditional English unit of weight for bread. A quartern-loaf is made from a quartern [2] of flour. The finished loaf usually weighs about 4 pounds (1.81 kilograms); as a result a quartern is sometimes described as a weight of 4 pounds, somewhat in conflict with the definition of 1/4 stone given above. quarter tone a unit used in music to describe the ratio in frequency between notes. The quarter tone is 1/24 octave, the basic interval in a 24-tone scale. Two notes differ by a quarter tone if the ratio in their frequencies is 21/24 = 1.0293. quartet a unit of quantity equal to 4. quartile a statistical unit equal to 25 percentiles, or 1/4 of a ranked sample. See percentile. quarto [1] a traditional Italian unit of volume, equal to about 73.6 liters or 2.60 cubic feet. quarto [2] a traditional Portuguese unit of volume, not related to the Italian quarto nor to the English quart. The Portuguese quarto equals 2 oitavos, which is about 3.46 liters or 0.92 U.S. gallon. There are 16 quartos in a fanega, 124 in a pipa. quarto [3] in English, quarto is a page size; see -mo. quintal (q) [1] a traditional unit of weight in France, Portugal, and Spain. Quintal is also the generic name for a historic unit used in commerce throughout Europe and the Arab world for more than 2000 years. The unit began as the Latin centenarius, meaning "comprised of 100" because it was equal to 100 Roman pounds (libra pondo). The centenarius passed into Arabic as the cantar or qintar and then returned to Europe through Arab traders in the form quintal. The German zentner and English hundredweight are familiar forms of this same unit in northern Europe. The traditional French quintal equaled 100 livres (48.95 kilograms or 107.9 pounds), but today the word "quintal" in France usually means a larger metric unit (see next entry). The Spanish quintal is 100 libras (about 46 kilograms or 101 pounds). The Portuguese quintal is larger; it is equal to 128 libras (about 129.5 pounds or 58.75 kilograms). quintal (q) [2] a common metric unit of mass equal to 100 kilograms or approximately 220.4623 pounds. Notice that the metric ton is roughly equal to its non-metric predecessors, but the metric quintal is about twice the size of the traditional quintal. quintet a unit of quantity equal to 5. quire (qr) a traditional unit of quantity used for counting sheets of paper. The word is from Latin, meaning "by fours." A quire was originally comprised of 24 sheets cut from four of the large sheets produced by the paper maker. In modern use a quire is often reckoned as 25 sheets, so that a ream of 20 quires is now 500 sheets rather than the traditional 480. Q unit a unit of heat energy equal to one quintillion (1018) Btu, 1000 quads, or about 1.055 zettajoules (ZJ). R R a commercial unit used to measure the effectiveness of thermal insulation. A thermal insulator is a material, manufactured in sheets, which resists conducting heat energy. Its thermal conductance is measured, in traditional units, in Btu's of energy conducted times inches of thickness per hour of time per square foot of area per °F of temperature difference between the two sides of the material. The R value of the insulator is defined to be 1 divided by the thermal conductance. This means R is an abbreviation for the complex unit combination hr·ft2·°F/in·Btu. In SI units, an R value of 1 equals 6.9335 meter kelvins per watt (Km/W). rad (rd) a metric unit measuring radiation dose. One rad is equal to a dose of 0.01 joule of energy per kilogram of mass, or 100 ergs of energy per gram of mass. The SI unit of radiation dose is the gray; one rad equals 0.01 gray. "Rad" is an acronym for "radiation absorbed dose." radian (rad) a unit of angle measure widely used in mathematics and science. One radian is 1/(2 pi) of the circle, or approximately 57.295 779? Using radians to measure angles seems unnatural at first. However, when angles are stated in radians the constant pi tends to disappear from the equations, and this greatly simplifies calculation. For example, the length of an arc is simply its radius multiplied by its angular measure in radians, and the area of a sector of a circle is simply its angular measure in radians multiplied by the square of the radius. The radian was defined and named by James Thomson in 1873. Thomson was a mathematics professor at Queens College, Belfast, Northern Ireland, and the brother of the famous physicist William Thomson, Lord Kelvin. radiation unit an older name for the becquerel. rai a traditional unit of land area in Thailand, still used in descriptions of real estate. The rai equals approximately 0.395 acre or 0.160 hectare. Rankine an absolute temperaure scale; see degree Rankine. ratel see rotl (below). rayl one of two units of sound impedance. When sound waves pass through any physical substance the pressure of the waves causes the particles of the substance to move. The sound impedance is the ratio between the pressure and the particle velocity it produces. The impedance is 1 rayl if unit pressure produces unit velocity. In MKS units, this means 1 rayl equals 1 pascal-second per meter (Pa·s/m), or (equivalently) 1 newton-second per cubic meter (N·s/m3). Confusingly, the same name, rayl, is used for the corresponding CGS unit, 1 dyne-second per cubic centimeter (dyn·s/cm3). The MKS rayl equals 0.1 CGS rayl. The units are named for Robert John Strutt, the fourth Lord Rayleigh (1875-1947). rayleigh (R) a CGS unit of light intensity used in astronomy and physics to measure the brightness of the night sky, auroras, etc. One rayleigh represents the light intensity of one million photons of light per square centimeter per second. A dark night sky has a light intensity of roughly 250 rayleighs. The unit honors the English mathematician and physicist John William Strutt, the third Lord Rayleigh (1842-1919). ream (rm) a traditional unit of quantity used for counting sheets of paper. The word is thought to be derived from the Arabic rizmah, meaning a bundle. A ream is equal to 20 quires, which would be 480 sheets with the traditional definition of a quire as 24 sheets. In recent years, however, the ream has been redefined to equal 500 sheets. (Working backwards, this changes the definition of a quire from 24 to 25 sheets.) The new definition reflects the current practice of marketing many kinds of paper in packages of 500 sheets. R?umur a temperature scale; see degree R?umur. recommended dietary allowance (RDA) units used in the U.S. to measure the amounts of certain nutrients found in foods or provided by supplements such as vitamin tablets. Each nutrient has its own RDA unit. Link: RDA table from Nutrition Health Reports. redshift (z) a unit of relative distance used in astronomy. The universe is expanding, so distant galaxies are receding from the earth. The faster the speed of recession, the farther the object. Just as sound from a receding train is lowered in pitch, light from distant galaxies is shifted toward longer wavelengths, that is, toward the red end of the spectrum. The redshift equals z if the wavelength of light is z + 1 times the normal wavelength; thus a redshift of 0.40 means that the wavelength of the light is 40% longer than normal. Using the Hubble space telescope, astronomers have measured redshifts greater than 5.0. register ton a unit of cargo capacity equal to 100 cubic feet; see ton [3]. rehoboam a large wine bottle holding about 4.5 liters, 6 times the volume of a regular bottle. The "h" is silent in English pronunciation. rem a unit used for measuring the effective (or "equivalent") dose of radiation received by a human or some other living organism. One rem is equal to the actual dose received, in rads (see above), multiplied by a "quality factor" which is larger for more dangerous forms of radiation. The rem is related to the rad in the same way that the sievert is related to the gray. "Rem" is an acronym for "roentgen equivalent: man," meaning that it measures the biological effects of ionizing radiation in humans. reputed quart see quart [2]. res or RES symbol for "resolution," a unit defined to be the number of dots or pixels per millimeter in an image. The unit is often stated before the measurement. RES 1 is equal to 25.4 dots per inch (dpi). retinol equivalent (RE) a unit of dosage for retinol (vitamin A) and for related substances such as beta carotene. One RE is equivalent to 5 international units (IU), or 1.5 micrograms, of retinol. U.S. nutritional authorities recommend that an adult diet provide 1000 RE per day. revenue ton or tonne (RT) a unit used for billing in the shipping industry. The size of a shipment in revenue tons is the number of metric tons or the number of cubic meters in the shipment, whichever is larger. revolution (r) a unit of angle measure, equal to a full circle, 360? or 2 pi radians. revolution per minute (r/min or rpm) a unit of frequency, used particularly for rotation rates in mechanics. One rev/min equals 1/60 hertz or 0.104 720 radian per second. The tachometers on auto dashboards are usually calibrated in units of 1000 r/min. reyn a unit of dynamic viscosity in the customary English system. See poise for a description of dynamic viscosity. With force measured in pounds of force (lbf), one reyn equals 1 lbf·sec/in2, approximately 14.881 64 poise, or 1.488 164 pascal second. The unit was named for a British scientist, Osborne Reynolds (1842-1912). Rhine foot or Rheinfuss the foot unit, or fuss, traditionally used in western and northern Germany. rhm a unit used in physics to measure the strength of gamma rays, a form of high-energy radiation emitted by some radioactive substances. A source of strength 1 rhm produces ionization at the rate 1 roentgen per hour at a distance of 1 meter from the source. The letters "rhm" stand for roentgen-hour-meter. ri a traditional Japanese unit of distance, sometimes called the Japanese league because it is of similar length to the European league. The ri equals 2160 ken or 12 960 shaku (the shaku being the Japanese equivalent of the foot). This is about 3927 meters or 2.44 statute miles. Richter scale a logarithmic scale measuring earthquake intensity. See magnitude [2]. rick a traditional unit of volume for firewood. A rick represents a stack of split firewood 4 feet high and 8 feet long, the logs being of a standard length, usually 16 inches. This is equivalent to 1/3 cord or 1.208 steres. However, because the size of a rick has been manipulated by vendors, it is illegal to sell firewood by the rick in several U.S. states. A rick is sometimes called a face cord. The name of the unit comes from a old Norse word for a stack of wood. ridge a traditional Welsh unit of distance equal to 3 leaps or 20 feet 3 inches (6.1722 meters). Riga last a traditional British unit of volume used for measuring timber. The Riga last is named for the Latvian capital, Riga, which was a major port for the shipment of timber from Russian forests. A Riga last is 80 cubic feet (2.265 cubic meters) of square-sawn timber or 65 cubic feet (1.841 cubic meters) of round timber. See also last. right angle a common unit of angle measure equal to 90? 100 grads, or 1/4 circle. RMS an abbreviation for root mean square (see below), a mathematical technique for averaging the values of a changing quantity. rod (rd) a traditional unit of distance equal to 5.5 yards (16 feet 6 inches). The English system combines units originating in several eras. The rod and the furlong were units used by the Anglo-Saxon residents of England before the Norman conquest of 1066. "Rod" is a Saxon word which meant just what it means today: a straight stick. The Norman kings accepted this unit but preferred to call it a perch (a word of French origin, meaning a pole; see perche) or simply a pole. The rod was originally equal to exactly 15 feet, but these were Saxon feet, 10% longer than modern foot. When the shorter foot became established in the thirteenth century, the royal government did not want to change the rod, because it was the basis of land measurement, land records, and taxes. Therefore the rod was redefined to equal 16.5 of the new feet. The relationship between the rod and the other English distance units was confirmed by the Parliamentary statute of 1592, which defined the statute mile to be either 320 rods or 1760 yards, thus forcing the rod to equal exactly 5.5 yards. By accident, this makes the rod just a bit longer than 5 meters, 5.0292 meters to be exact. Thus 100 meters is very nearly 20 rods. roede a traditional Dutch unit of distance (see rood [1] below), now reinterpreted as a metric unit equal to exactly 10 meters (32.8084 feet). The roede has also been used as a unit of area equal to one square (linear) roede; this is equal to 100 square meters or 1 are. roentgen or r?tgen (R) a non-metric unit used to measure the ionizing ability of radiation. Radiation often ionizes atoms it strikes, stripping one or more electrons from them. The biological effects of radiation are caused in large part by excessive ionization within living cells, so it is important to measure this ionizing ability of radiation. For x-rays and gamma rays, this is often done by measuring the electric charge released when air is ionized by the radiation. The roentgen is an old unit used for this purpose; one roentgen equals a charge release rate of 258 microcoulombs per kilogram of air. The unit is named for one of the early investigators of radioactivity, the German physicist Wilhelm Konrad R?tgen (1845-1923). rood [1] an old unit of distance, used in several ways. Rood (or roede) is an old Dutch word meaning a rod or pole. So the rood is in some cases another name for a rod. But in old England and Scotland the rood was often longer than a "modern" rod of 16.5 feet; sometimes it was 20 feet, 21 feet, or even 24 feet. In Afrikaans-speaking South Africa, the rood was a standardized measure equal to 12.396 ft or 3.7783 meters. rood [2] a traditional unit of area used to measure land. The rood is defined to be 40 square rods, which equals 1210 square yards, or 10 890 square feet, or exactly 1/4 acre. That would be the area of a lot 22 yards wide and 55 yards deep, about the size of many suburban lots. One rood is approximately 1011.714 square meters, or 0.101 171 4 hectare. root mean square (RMS) a notation used after various measurements to indicate that the root mean square method has been used to measure or compute an average value for the measurement. Usually the quantity being measured varies in a periodic way; typical examples include the voltage of an alternating current or the intensity of a sound wave. In the RMS method, the varying quantity is first squared (S), then a mean (M) or average of the squared values is obtained, and then the square root (R) of this mean value is computed. For many purposes this procedure gives the best measure of the "typical" or "effective" value of the quantity. rope another name for the rood [1], the distance unit. rotl, rotel, rottle, ratel, or arratel a traditional Arab unit of weight corresponding to the Roman libra, the French livre, and the English pound. There was considerable variation in the unit from time to time and from place to place, but usually the rotl was about 0.9-1.15 pound (450-530 grams). However, in some areas of the Near East, such as Syria and Palestine, larger rotls of 5.5 to 6 pounds (2.5-2.8 kilograms) were used. This unit has many spellings in European languages. royal foot the French pied de roi, also called the Paris foot in English. run a unit of density for woolen yarn, used in the U.S. Yarn is described as n run if there are n 1600-yard hanks of the wool per pound. Actual yarn ranges from about 0.5 run to 8 run. The unit is also called the American run. rundlet a traditional measure of liquid volume, dating back to the Middle Ages. A rundlet is a small barrel usually holding18 wine (U.S.) gallons (roughly 68.1 liters). running foot another name for a linear foot. rute a traditional German unit of distance corresponding to the English rod. In fact, "rute" is the German word for rod. The rute had varying lengths, as short as 10 f?se (German feet) and as long as 16 f?se. This could be anywhere from about 3 meters to 4.5 meters. rutherford (Rd) a practical unit of radioactivity equal to the megabecquerel (MBq). This means 1 rutherford represents 1 million radioactive disintegrations per second. The unit is named for the British nuclear physicist Ernest Rutherford, later named Lord Rutherford (1871-1937), whose study of radioactivity led to the discovery in 1911 that most of the mass of an atom is concentrated in a tiny nucleus. S sabin a non-metric unit of sound absorption used in acoustical engineering. One sabin is the sound absorption of one square foot of a perfectly absorbing surfacesuch as an open window! The sound absorption of a wall or some other surface is the area of the surface, in square feet, multiplied by a coefficient which depends on the material of the surface and also on the frequency of the sound. These coefficients are carefully measured and tabulated. The unit honors Wallace Sabine (1868-1919), a Harvard University professor who founded the systematic study of acoustics about 1895. Sabine used this unit, which he called the open window unit (owu), as early as 1911. sack [1] a traditional unit of volume. Sacks of different commodities are of different sizes, but a typical measure is 3 bushels (about 105.7 liters based on the U.S. bushel, or 109.1 liters based on the British Imperial bushel). sack [2] a traditional unit of weight, varying for different commodities shipped in sacks. For example, in the U.S., a sack of salt was traditionally equal to 215 pounds, a sack of cotton 140 pounds, and a sack of flour 100 pounds. sadzhen or sagene a traditional Russian unit of distance corresponding roughly to the English fathom and French toise. Early in the 1700s, Czar Peter the Great fixed the length of the sadzhen at exactly 7 English feet. This equals 3 arshin or 2.1336 meters. Sagene is an older transliteration of the Russian word. Saffir-Simpson category a ranking of the strength of a hurricane, introduced by two American meteorologists and used by the U.S. National Weather Service to rank storms in the Atlantic and northeastern Pacific Oceans. A complete description, from the U.S. Atlantic Oceanographic and Meteorological Laboratory in Miami, is provided. salmanazar a large wine bottle holding about 9 liters, 12 times the volume of a regular bottle. saltspoon (ssp) a unit of volume formerly used in U.S. food recipes. The saltspoon equals 1/4 teaspoon or about 1.2 milliliters. saros a unit of time used in astronomy, mostly in predicting solar and lunar eclipses. The saros is equal to 6585.32 days (6585 days 7 hours 23 minutes), which is exactly 223 lunar months. (This is either 10 or 11 days more than 18 years, depending on the number of leap years during the period.) Astronomers in ancient times discovered that the saros is very nearly equal to 19 eclipse years (6585.78 days). This means that one saros after an eclipse the Sun, Moon, and Earth return almost exactly to the same position and another, very similar eclipse occurs. However, because of the 7 hours 23 minutes included, the Earth has turned about one third of a revolution and the new eclipse occurs about 116?of longitude west of the preceding one. After 3 saros, the eclipse returns nearly to its original location. Thus eclipses at a particular location tend to repeat with a period of 3 saros or 54 years 1 month. Thus the last total solar eclipse in North Carolina, on 1970 March 7, will repeat on 2024 April 8 and again on 2078 May 11. savart a unit used in music to describe the ratio in frequency between notes. There are 301 (sometimes 300) savarts in an octave. This means two notes differ by one savart if the higher note has a frequency equal to 21/301 = 1.002 305 times the frequency of the lower note. The unit is named for a French physicist, F?ix Savart (1791-1841); although best known for his work in electromagnetism, Savart also did pioneer research in the physics of sound. sccm, scfm, scfd symbols for "standard cubic centimeters per minute", "standard cubic feet per minute," and "standard cubic feet per day." These are units of flow rate for gases, and the term "standard" indicates that the flow rate assumes standard temperature (0°C) and standard pressure (1 atmosphere). The units sccm and scfm often appear as ratings on devices for controlling the flow of gases. The unit scfd is the standard unit of natural gas production in the petroleum industry. scheffel or schepel traditional units of dry volume. The German scheffel and Dutch schepel have both been redefined within the metric system, but in very different ways: the scheffel equals 50 liters (1.4189 U.S. bushels) and the schepel 10 liters (0.2838 U.S. bushels). Both words are usually translated "bushel" in English, and both units were originally closer to the English bushel; the schepel was roughly 0.75 bushel or about 26 liters. See also skep (below). schooner an informal unit of liquid volume. A schooner is a large tumbler or drinking glass holding about 400 milliliters or 13.5 U.S. fluid ounces. Similarly, in Queensland and New South Wales (Australia) a schooner of beer holds 425 milliliters. In South Australia, however, a schooner is only 285 milliliters. schoppen a traditional German unit of liquid volume now interpreted as 375 milliliters (0.5 bottle or 12.68 U.S. fluid ounces). schtoff a traditional Russian unit of volume equal to 10 charki. This is equivalent to about 1.23 liters or 1.30 U.S. liquid quarts. score a traditional unit of quantity equal to 20. The score, like the dozen, helps us describe a moderate number of objects. This is one of the many cases in which English has two words for similar concepts, one word from the Old French spoken in 1066 by the Norman conquerors of England and one from the Old English spoken by the Anglo-Saxon people they conquered. In this case, dozen is the French word and score is the Old English word, derived from the Norse word skor meaning a notch cut in a stick as a tally mark. Scots foot, Scots mile traditional distance units in Scotland. The Scots foot equals 1.005 405 4 English foot (about 30.645 centimeters). The Scots mile equals 320 falls or 5920 Scots feet, which is about 5952 English feet (1.127 English mile or 1814.2 meters). The English statute mile, of course, is now official in Scotland. Scoville unit a unit measuring the concentration of capsaicin, the "hot" ingredient in chile peppers. A measurement of, say, 50 000 Scoville units means that an extract from the pepper can be diluted 50 000 to 1 with sugared water and the "burn" of the capsaicin will still be barely detectable by the human tongue. (In practice, the measurements are now made with liquid chromatography.) The unit was invented in 1912 by the American pharmacologist Wilbur L. Scoville, who was working on the use of capsaicin in the muscle pain-relieving ointment Heet. Actual chile peppers have capsaicin concentrations from 5000 to 500 000 Scoville units. scruple (s) a unit of weight in the traditional system used by English apothecaries, equal to 20 grains or approximately 1.2960 gram. See also dram [2]. The name of the unit is from the Latin scrupulus, meaning a small, sharp stone. (This word also came to mean "something which causes pain or annoyance," such as a pebble in one's shoe, and this led to our other use of the word "scruple" to mean an ethical consideration.) Units similar to the scruple were used throughout Europe; the French and Russian scrupule, Italian scrupolo, and German skrupul are equal to 20 of the local unit corresponding to the grain and all are equivalent to something in the range 1.1-1.3 grams. se a traditional Japanese unit of area equal to about 99 square meters or 118.4 square yards. seam a traditional unit of volume. A seam of grain was 8 bushels: this would be equivalent to 290.95 liters based on the British Imperial bushel, or 281.91 liters based on the older U.S bushel. Take your pick. The old English word "seam" meant the load of a pack animal, and that is the origin of the volume unit. The same word also came to mean the stitching of the packsaddle, and that's the origin of our other uses of the word today. second (s or sec or ") [1] the fundamental unit of time in all measuring systems. The name simply means that this unit is the second division of the hour, the minute being the first. The second was defined as 1/86 400 mean solar day until astronomers discovered that the mean solar day is actually not constant (see day). The definition was then changed to 1/86 400 of the mean solar day 1900 January 1. Since we can't go back and measure that day any more, this wasn't a real solution to the problem. In 1967, scientists agreed to define the second as that period of time which makes the frequency of a certain radiation emitted by atoms of cesium-133 equal to 9 192 631 770 hertz (cycles per second). In other words, if we really want to measure a second, we count 9 192 631 770 cycles of this radiation. This definition allows scientists to reconstruct the second anywhere in the world with equal precision. second (" or s or sec) [2] a unit of angular measure equal to 1/60 arcminute. This unit is also called the arcsecond to distinguish it from the second of time. One second is a very small angle indeed: there are 1 296 000 seconds in a circle. The SI defines s as the symbol for the time unit (see above) and recommends " as the symbol for the arcsecond. second (" or s or sec) [3] a unit of longitude used in astronomy. Astronomers measure longitude (or, as they call it, right ascension) in time units by dividing the equator into 24 hours instead of 360 degrees. This makes 1 second of longitude equal to 15 arcseconds. second (" or sec) [4] a unit of viscosity defined by the rate at which a liquid flows through a particular viscometer. The Saybolt Universal second was used in the U.S., the Redwood second in Britain, and the Engler second in continental Europe. Generally speaking, these readings are tied to specific instruments and can't be reduced to absolute units without the use of an appropriate table. section (sec) a traditional unit of area in the U. S., equal to 1 square mile. This unit is used mostly in the midwestern states, where public land was originally surveyed into sections one mile square. seemeile or sea mile another name for the nautical mile. Seemeile is the customary name in German. seer [1] a traditional weight unit in India and South Asia. The seer equals 1/40 maund, and, like the maund, it varied considerably from one area to another. The official size in British India was 2.057 15 pounds or 0.9331 kilogram. In Pakistan, the seer is now considered equal to the kilogram. seer [2] a traditional unit of dry volume in northern India, equal to a little more than a liter. This is roughly the volume of a seer [1] of grain. seidel a traditional unit of liquid volume in Austria. The traditional seidel equals about 354 milliliters; this is about 12.0 U.S. fluid ounces or about 12.5 British fluid ounces. The seidel has also been used as a metric unit equal to 300 milliliters. semester (sem) an informal unit of time. The word semester comes from the Latin words for "six months," and originally a semester was understood to equal 6 months or 1/2 year. However, the word is now used chiefly to mean half the academic year at a school or college, a period of time which can vary from 15 to 21 weeks. semester hour (sem hr) a unit of academic credit, supposedly equal to one semester's study for a period of one hour per school day. In U.S. secondary schools, this means 5 hours per week, and in U.S. colleges and universities, usually 3 hours per week. However, "academic hours" slightly shorter than regular hours (often 50 or 55 minutes per class) are typically used in these calculations. semicircle a unit of angle measurement equal to 1/2 circle, pi radians, or 180? semitone a unit used in music to describe the ratio in frequency between notes. The unit is actually used in two slightly different ways. In one use, two notes are said to differ by one semitone if the higher note has frequency exactly 16/15 = 1.0667 times the frequency of the lower one. Also, the semitone is used as a synonym for the half step in the standard chromatic scale; in this use, two notes differ by a semitone if the higher note has frequency exactly 21/12 = 1.0595 times the frequency of the lower one. septet a unit of quantity equal to 7. seventh a unit used in music to describe the ratio in frequency between notes. Two notes differ by one seventh if the higher note has frequency exactly 15/8 times the frequency of the lower one. On the standard 12-tone scale, the seventh is approximated as 11 half steps, corresponding to a frequency ratio of 211/12 = 1.8877. sextet a unit of quantity equal to 6. shackle a traditional unit of length used for measuring the lengths of nautical cables and chains. The size of the unit varied. In the most recent version, adopted in 1949 by the British Navy, the shackle was defined to equal 15 fathoms, which is 90 feet or 27.432 meters. shaftment an old English unit of distance equal to 2 palms or 6 inches (15.24 centimeters). A shaftment is the distance from the tip of the outstretched thumb to the opposite side of the palm of the hand. The ending "-ment" is from the old English word mund, hand. shake an informal unit of time equal to 10-8 second or 10 nanoseconds (ns). shaku the Japanese foot, equal to about 30.30 centimeters or 11.93 inches. shannon (Sh) a unit of information content used in information and communications theory. The definition is based on the idea that less-likely messages are more informative than more-likely ones (for example, if a volcano rarely erupts, then a message that it is erupting is more informative than a message it is not erupting). If a message has probability p of being received, then its information content is -log2 p shannons. For example, if the message consists of 10 letters, and all strings of 10 letters are equally likely, then the probablity of a particular message is 1/2610 and the information content of the message is 10(log2 26) = 47.004 shannons. This unit was originally called the bit [3], because when the message is a bit string and all strings are equally likely, then the information content turns out to equal the number of bits. One shannon equals log10 2 = 0.301 030 hartley. The unit is named for the American mathematician Claude Shannon (b. 1916), the founder of information theory. sheaf a traditional measure for grain and various other commodities. A sheaf of grain is a bundle of stalks having a standard size, typically 30 to 36 inches (roughly 75-90 centimeters) in circumference. A sheaf of arrows is a bundle or quiver of 24. shed an exceptionally small unit of area used in particle physics. Like its big brother, the barn, the shed is used to express the apparent cross-sectional area of a particle from which other particles are scattered. One shed equals 10-24 barn, which is 10-52 square meter or 0.0001 square yoctometer (ym2). shekel an ancient Hebrew unit of weight (and also a coin having that weight). The shekel was the Hebrew version of a Babylonian unit used throughout the Middle East. Historians differ somewhat on its size. A frequently quoted equivalent is 252 grains, which is equal to 0.5760 ounce (avoirdupois) or about 16.33 grams. sheng a traditional unit of liquid volume in China. Like the Indian seer (see above), the sheng is a little more than a liter; 1.035 liter (1.094 U.S. quart) is one quoted equivalent. sho a traditional Japanese unit of liquid volume. The sho equals 1.8039 liter, which is 1.9061 U.S. quarts or 1.5872 British imperial quarts. shock or shook [1] a unit of quantity equal to 5 dozen, 3 score, or 60. The unit is more common in German, where the word is spelled schock, than it is in English. In cooperage (the making of barrels), a bundle of 60 barrel staves is traditionally called a shock. shock or shook [2] a traditional measure of grain or straw. A shock of grain is usually 12 sheaves (see above), sometimes 10. short hundredweight, short ton common names, respectively, for the U.S hundredweight or cental (100 pounds) and the U.S. ton (2000 pounds). shot a traditional unit of liquid volume. The term "shot" is often used informally to mean "a small serving." In the U.S. a shot is legally equal to one fluid ounce or 29.574 milliliters. However, many bartenders use shot glasses holding the same as a jigger: 1.5 fluid ounces or 44.4 milliliters. shower unit a unit of distance used in studying cosmic radiation; it is the distance a charged particle travels in a given material while its energy decreases by 50%. Thus it depends on the material: in air the shower unit is very long, about 230 meters, but in water it is only 30 centimeters. sidereal day a unit of time used in astronomy, equal to the period of time in which the earth makes one rotation relative to the stars. If we could view the earth from outside the Solar System, we would see that it actually completes 366 rotations during one year (one revolution around the sun). We only count 365 because one rotation is cancelled out for us by our tour around the sun. Thus the sidereal day, the average interval between two successive risings of the same star, is a little shorter than the mean solar day (see day). The sidereal day equals 23 hours 56 minutes 4.09 seconds, or 86 164.09 seconds. Like the regular day it is divided into 24 sidereal hours, each sidereal hour being divided into 60 sidereal minutes and each sidereal minute into 60 sidereal seconds. Clocks in observatories are set to this sidereal cycle. Siegbahn unit another name for the X unit, a unit of distance used for describing the wavelength of X rays. siemens (S) the SI unit of electric conductance. The most important property of a conductor is the amount of current it will carry when a voltage is applied. A conductor has a conductance of one siemens if it carries one ampere of current per volt of potential. Conductance is the inverse of resistance, and the siemens is the reciprocal of the ohm. In other words, the conductance, in siemens, is simply 1 divided by the resistance, in ohms. The unit is named for the German electrical engineer Werner von Siemens (1816-1892). sieve a traditional measure of the fineness of a wire screen (such as the screening used in a sieve). Higher sieve numbers correspond to finer screens. In The U.S., 10 sieve fabric has openings of 2 millimeters; 100 sieve has openings of 0.15 millimeters. A similar but slightly different scale was used in Britain. Link: sieve comparison table from Screen Technology Group, Inc. sievert (Sv) an SI unit used for measuring the effective (or "equivalent") dose of radiation received by a human or some other living organism. Various kinds of radiation have different effects on living tissue, so a simple measurement of dose as energy received, stated in grays or rads, does not give a clear indication of the probable biological effects of the radiation. The equivalent dose, in sieverts, is equal to the actual dose, in grays, multiplied by a "quality factor" which is larger for more dangerous forms of radiation. An effective dose of one sievert requires 1 gray of beta or gamma radiation but only 0.05 gray of alpha radiation or 0.1 gray of neutron radiation. One sievert equals 100 rem. The sievert is a large unit, so radiation doses are often measured in millisieverts (mSv). The unit honors the Swedish physicist Rolf Sievert (1898-1966), who worked over many years to measure and standardize the radiation doses used in cancer treatment. sign an informal unit of angle measure originating in astrology. The Sun's annual path through the sky, called the Zodiac, is divided into 12 parts called signs, each sign corresponding roughly to one of the classic twelve constellations through which the Sun passes. Thus 1 sign is equal to 1/12 circle, or 30? sixth a unit used in music to describe the ratio in frequency between notes. Two notes differ by one minor sixth if the higher note has frequency exactly 8/5 times the frequency of the lower one, or by a major sixth if the higher note has frequency exactly 5/3 times the frequency of the lower one. On the standard 12-tone scale the minor sixth is approximated (roughly) by 8 half steps, corresponding to a frequency ratio of 22/3 = 1.5874; the major sixth is approximated by 9 half steps, corresponding to a frequency ratio of 23/4 = 1.6818. skein a coil of yarn or cord. In retail trade, a skein is a highly variable unit, varying from one type of yarn to another and often from one manufacturer to another. In textile manufacturing, however, the skein is a unit of length equal to the lea. skep, sk?pe, skjeppe a traditional unit of dry volume in northern Europe. Use of the unit was spread by Norse traders. In Britain, the skep was eventually identified with the bushel and the word survives as a name for a farm basket or a domed beehive. The Danish sk?pe and Norwegian skjeppe are equal to 18 pots; this is equivalent to 17.407 liters (0.494 U.S. bushel) in Denmark and 17.370 liters (0.493 U.S. bushel) in Norway. The Dutch schepel and German scheffel (see above) are versions of this same unit. skot a unit of (low) luminance equal to 0.001 apostilb. This unit was used in Germany during World War II to describe permitted levels of lighting during air raids. One skot is the brightness of a surface produced by 1 nox or 0.001 lux of light. The name comes from the ancient Greek skotos, darkness. slug a unit of mass in the English foot-pound-second system. One slug is the mass accelerated at 1 foot per second per second by a force of 1 pound. Since the acceleration of gravity (1 g) in English units is 32.174 04 feet per second per second, the slug is equal to 32.174 04 pounds (14.593 90 kilograms). The slug was formerly used in calculations in mechanics and engineering, but it has been largely abandoned in favor of metric units. Snellen fraction a ratio (such as 20/20 or 20/100) measuring the acuity (sharpness) of a person's eyesight. The denominator (number after the slash) is the distance at which the detail of a letter on a standard test chart would subtend an angle of one arcminute, and the numerator (number before the slash) is the testing distance, the distance at which the person correctly identifies the letter. By custom, the ratio is usually stated with a testing distance of 20 feet or 6 meters; thus 20/100 (or, in metric countries, 6/30) means that the person located 20 feet (6 meters) from the chart can identify a letter subtending an angle of one arcminute at a distance of 100 feet (30 meters). The unit is named for the Dutch opthalmologist Herman Snellen (1834-1908), who developed the first standard methods for measuring visual acuity. snit a unit of volume for liquor equal to 2 jiggers, 3 U.S. fluid ounces, or 88.7 milliliters. sol a unit of time on Mars, equal to the average length of the Martian day as it would appear to an observer on the red planet. The sol equals 24 hours 39 minutes 35 seconds, which is 88 775 seconds or about 1.0275 Earth days. solar neutrino unit (snu) a unit used by astrophysicists to measure the rate at which neutrinos from the Sun are detected on Earth. Neutrinos are elementary particles with no charge and little or no mass (in 1999 it was reported that neutrinos do have a small mass). Scientists want to detect neutrinos because they carry crucial information about processes deep inside the Sun, but neutrinos are very hard to catch: most of them pass right through the Earth without interacting with anything. The solar neutrino unit, therefore, is extremely small: it is defined to be 10-36 neutrino capture per target atom per second. -some a suffix added to a number to create a unit of quantity. For example, a foursome is a group of four. The suffix comes from the use of "some" as a pronoun meaning a number of individuals, as in "Some Like It Hot." sone a unit of subjective sound loudness. One sone is the loudness of a pure tone of frequency one kilohertz and strength 40 decibels. A sound has loudness s sones if a listener judges it to be s times louder than a sound of 1 sone. The phon is another unit of sound loudness; a sound of loudness p phons has loudness 2(p - 40)/10 sones. The sone is often used in industrial engineering to express the perceived loudness of engines, fans, and other items of industrial equipment. The unit was introduced by S.S. Stevens and H. Davis in 1938. Its name is from the Latin word sonus, sound. span a traditional unit of distance, equal to 9 inches (approximately 22.9 centimeters) or 1/4 yard. This distance represents the span of a man's hand with fingers stretched out as far as possible. The Old English word spann meant precisely this unit of measure; the other uses of the word "span" came later. sphere the traditional unit of solid angle measure, divided into 4(pi) steradians (see below). There are also 129 600/(pi) = 41 252.96 square degrees in a sphere. spin a unit of angular momentum used in particle physics. The spin unit is equal to Planck's constant h divided by 2pi, or approximately 105.457 27 x 10-36 joule second. The spin of an elementary particle is always a simple multiple of this unit. spindle a traditional measure of length used for yarn. The length varied with the material; a spindle of cotton yarn, for example, was 15 120 yards (13.826 km), and a spindle of jute was 14 400 yards (13.167 km). The cotton spindle was also equal to 18 hanks. split a unit of volume for liquor equal to 4 jiggers, 6 U.S. fluid ounces, or 177.4 milliliters. The same name is sometimes used for a small wine bottle holding 1/4 the volume of a regular bottle; this would be about 187.5 milliliters. spoonful an informal unit of volume, sometimes an alternate name for the teaspoon. square (sq) a traditional unit of area used for measuring roofing material, finished lumber, and other building materials. One square equals 100 square feet or about 9.290 square meters. square centimeter (cm2) the CGS unit of area. 1 cm2 equals 100 mm2, 10-4 m2, or approximately 0.155 000 3 in2. square chain (ch2 or sq ch) a traditional unit of area in English surveying, defined to be the area of a square whose side equals Gunter's chain (4 rods or 22 yards). Thus the square chain equals 16 square rods, 484 square yards, 4356 square feet, or exactly 0.1 acre. In metric terms, the square chain equals about 4.0569 ares, or 405.69 square meters. square degree (sq deg) a unit of solid angle measure. Since one degree of ordinary angle measure equals pi/180 radians,one square degree is defined to be (pi/180)2 = 0.000 304 617 4 steradian. There are 4?802 = 129 600/(pi) = 41 252.961 25 square degrees in a sphere. square foot (ft2 or sq ft) a traditional unit of area. 1 ft2 equals 144 in2 or exactly 929.0304 cm2 (0.092 903 04 m2). square inch (in2 or sq in) a traditional unit of area. 1 in2 equals exactly 6.4516 cm2 or 645.16 mm2. square kilometer (km2) a common metric unit of area. One square kilometer equals 100 hectares, 106 square meters, approximately 247.105 acres, or approximately 0.386 102 square mile. square meter (m2) the SI unit of area, equal to 104 square centimeters, approximately 10.763 91 square feet, or approximately 1.195 990 square yard. square mile (mi2 or sq mi) a traditional unit of area. One square mile is equal to 640 acres, 3 097 600 square yards, 258.9988 hectares, or 2.589 988 square kilometers. square yard (yd2 or sq yd) a traditional unit of area. 1 yd2 equals 9 ft2, 1296 in2, or exactly 0.836 127 36 m2. stack (stk) a traditional unit of volume used in Britain for coal and firewood. The stack equals 4 cubic yards, which is 108 cubic feet or about 3.058 cubic meters. As a firewood measure, a stack equals 27/32 = 0.843 75 cord. stade, stadion or stadium a historic unit of distance originating in ancient Greece. Greek athletic fields were all of roughly the same size, and the stadion, as a unit of measure, represented the length of the field. Archaeological measurements show that the stadion was roughly 200 yards or a little less than 200 meters. The stadion at Olympus, where the original Olympic Games were held, measures 630.8 feet or 192.3 meters. Stadium is the Latin spelling; in the Roman world the stadium was equal to 625 Roman feet (peda) or 1/8 Roman mile. This is equivalent to 606.95 feet, 202.32 yards, or 185.00 meters. The plural is stadia. standard (std) a traditional unit of volume used in northern Europe to measure the volume of finished lumber. The standard equals 165 cubic feet, which is equivalent to 1980 board feet or about 4.672 cubic meters. standard deviation (sd) a mathematical unit used to describe the "spread" or dispersion of a set of data. Each item in the data set has a deviation from the mean (the ordinary average) of the data. The standard deviation is computed by taking the squares of these individual deviations, adding them, and then taking the square root of the sum. If the data set conforms to a known distribution, such as the normal ("bell curve") distribution, then one can compute the percentage of the data which will fall within a certain distance of the mean, as measured in standard deviations. For example, if the data set conforms to the normal distribution, 68.3% of the data will fall within one standard deviation of the mean and only about 4.5% will fall outside 2 sd from the mean. standard volume a unit sometimes used by chemists and physicists to measure the volumes of gases. The behavior of gases under ordinary conditions (not at very high pressures or very low temperatures) is governed by the Ideal Gas Law. This law says that the volume V of a gas is related to its temperature T and pressure P by the formula PV = nRT, where n is the number of moles of gas present and the gas constant R equals 8.314 joules per mole per kelvin. One standard volume is the volume one kilomole of gas occupies at standard temperature (273.16 kelvins, or 0°C) and standard pressure (1 atmosphere, or 101.325 kilopascals). The standard volume is equal to 22.414 cubic meters or 29.316 cubic yards. stang a traditional Welsh unit of land area generally equal to 3240 square yards, 0.6694 acre, or 0.2709 hectare. stapp a unit used to express the effects of acceleration or deceleration on the human body. One stapp represents an acceleration of 1 g for a period of 1 second, or 9.80665 meters per second per second for 1 second. The unit is named for the U.S. Air Force physician John P. Stapp (1910-1999), a pioneer in research on the human effects of acceleration during the 1940s and 1950s. stat- a prefix indicating that an electrical unit is part of the CGS electrostatic system. These units are also indicated by the notation esu (as in "volt esu"). Although these units are defined naturally as part of the CGS system, they are usually much too large or much too small for most applications. They have been replaced almost completely by the corresponding SI units. Following are the SI equivalents for each of the "stat" units: Electric current: 1 statampere = 3.3356 x 10-10 amperes Electric charge: 1 statcoulomb = 3.3356 x 10-10 coulombs Capacitance: 1 statfarad = 1.1126 x 10-12 farads = 1.1126 picofarads Inductance: 1 stathenry = 8.9876 x 1011 henrys Resistance: 1 statohm = 8.9876 x 1011 ohms Potential: 1 statvolt = 299.79 volts Power: 1 statwatt = 10-7 watts step [1] a traditional unit of distance, equal to 1/2 pace. The step is traditionally equal to 30 inches or 76.2 centimeters. step [2] a unit used in music to describe the ratio in frequency between notes. Two notes differ by a step if the higher note has frequency exactly 21/6 = 1.12246 times the frequency of the lower one. steradian (sr) the standard unit of solid angle measure in mathematics. Just as there are 2pi radians in a circle, there are 4pi steradians in a sphere. Thus one steradian equals about 0.079 577 sphere. There are 129 600/pi = 41 252.96 square degrees in a sphere, so 1 steradian also equals about 6565.619 square degrees. stere (st) a metric unit of volume, equal to one cubic meter or one kiloliter (about 35.3147 cubic feet or 1.307 95 cubic yards). Although it dates from the origin of the metric system in 1798, the stere has never been used as much as the liter. In Europe it is used primarily as a measure for firewood; for this purpose, the stere equals about 0.2795 cord or 2.207 cord feet. The name comes from the Greek stereos, solid. sthene (sn) a metric unit of force, part of the "metre-tonne-second" system sometimes used by European engineers. One sthene is the force required to accelerate a mass of one tonne at a rate of 1 m/s2. Thus the sthene is equal to the kilonewton, so 1 sthene is equivalent to 108 dynes, 224.809 pounds of force or 7233.01 poundals. The name comes from the Greek word sthenos, strength. stick (stk) an informal unit used to measure butter. In the United States, butter is usually sold in 1-pound packages containing 4 sticks; thus 1 stick equals 1/4 pound (roughly 113 grams). On the other hand, 1 stick is also considered equal to 1/2 cup (roughly 118 milliliters). Thus the unit is used both as a weight measure and as a volume measure. stilb (sb) a CGS unit of luminance, equal to one candela per square centimeter or 104 lux. The unit is believed to have been used first by the French physicist A. Blondel about 1920. The name comes from a Greek word meaning a lamp. stoke or stokes (St) a CGS unit of kinematic viscosity. Kinematic viscosity is defined to be dynamic viscosity (see poise) divided by the density of the liquid; this gives a quantity which depends only on the type of the liquid, independent of its concentration or density. The quotient turns out to have units length2/time. Being a CGS unit, the stokes is therefore defined to be 1 cm2/s, equivalent to 10-4 m2/s or 0.001 076 391 ft2/s. The SI has no named unit of kinematic viscosity, requiring the use instead of m2/s. The unit, called the stokes in Britain and the stoke in the U.S., is named for a British mathematician and physicist, Sir George Gabriel Stokes (1819-1903), who described the basic principles of fluid mechanics in 1845. stone (st) a traditional British unit of weight, rarely used in the U.S. Originally the stone varied in size, both from place to place and according to the nature of the item being weighed. Eventually it was standardized at 14 pounds avoirdupois or approximately 6.350 29 kilograms. This was a convenient size because it made the stone equal to exactly 1/2 quarter or 1/8 hundredweight. No -s is added for the plural. stop a unit of relative exposure used in photography. In optics, a "stop" is a ring-shaped baffle or shield used to reduce the aperture (the diameter of the opening) of a lens. In photography, the amount of light used to expose the film can be controlled either by varying the aperture, by varying the length of time the shutter is open to admit light, or by some combination of these two methods. Two exposures differ by one stop if one is made with twice the light of the other; similarly, they differ by n stops if one is made with 2n times the light of the other. story an informal unit of distance equal to the average distance between floors of a building. Typically a story equals 10 to 12 feet (3.0-3.6 meters). The origin of this use of the word "story" is not entirely clear, but in medieval times a tier of sculptures or stained glass windows on the front of a cathedral was called a story because it usually told a story, and the number of stories was a measure of the size of the building. stremma a traditional Greek unit of area, now redefined to equal exactly 1000 square meters or 0.1 hectare (0.24710 acre). stride another name for a pace. strike a traditional British unit of volume, varying considerably but usually more than a bushel. A convenient equivalent is 2 imperial bushels (roughly 2.5 cubic feet or 73 liters). strob a unit of angular velocity. The strob represents a rotation rate of one radian per second or 9.54930 rpm. The name comes from the Greek strobos, meaning "rotating." strontium unit a unit of radioactive concentration used to measure the presence of the very dangerous radioisotope strontium 90 in the body or elsewhere. One strontium unit is a concentration of 1 picocurie per gram (pCi/g). In SI units this is equal to exactly 37 bequerels per kilogram (Bq/kg), that is, 37 atomic disintegrations per kilogram per second. stunde the German word for the hour. In Switzerland, the stunde is also a unit of distance defined to be 4800 meters (2.983 miles) approximately the distance a person can walk in an hour. The definition is consistent with the custom everywhere of informally adopting time units as distance units, as when we say "I live 10 minutes from school." Sun a unit of mass used in astronomy to express the masses of stars. Stars are so massive that it's difficult to grasp their relative sizes. Using the Sun's mass as a unit helps us visualize them relative to something familiar. The best current estimate of the mass of the Sun is 1.9891 x 1030 kilograms. superficial an adjective used to convert a unit of length into a unit of area. "Superficial", in this use, means "square": a superficial foot, for example, is equal to a square foot, and a superficial yard is equal to a square yard. (In Australia, a superficial foot often means the super foot [2] (see below). super foot [1] a British commercial unit of area equal to one square foot. The name originated as an abbreviation (ft. super. or super. ft.) for the superficial foot. super foot [2] in Australia and New Zealand a super foot is a unit of volume for timber or lumber, equal to the volume of a board one foot square and one inch thick. This unit is the same as the North American board foot. svedberg (S) a unit equal to 10-13 second (0.1 picosecond), used to measure sedimentation coefficients in ultracentrifuge technology. The sedimentation coefficient of a particle is defined to be s/(w2r), where s is the sedimentation rate, w is the angular velocity of the centrifuge (in radians per second), and r is the radius at which the particle is being spun; this calculation gives a quantity in time units. The unit honors the Swedish chemist Theodor Svedberg (1884-1971), who received the Nobel Prize in Chemistry for 1926 for the invention of the ultracentrifuge and its use in studying the properties of colloids. sverdrup a unit of flow sometimes used in oceanography to express the flow of ocean currents. One sverdrup equals one million cubic meters per second, which is also one cubic hectometer per second. The unit honors the Norwegian oceanographer and Arctic explorer H. U. Sverdrup (1888-1957). SWU symbol for the separative work unit, a unit used in the nuclear power industry to describe the work required to enrich uranium (that is, to increase the fraction of uranium 235, the isotope which undergoes fission to produce energy). The details are quite complex, but roughly speaking about 4.5 SWU are needed to enrich 1 kilogram of uranium, more or less depending on the degree of enrichment needed for a particular reactor. Return to the Dictionary Contents page. T tablespoon or tablespoonful (tbsp, tblsp, or Tsp) [1] a unit of volume used in food recipes. In the U.S., the tablespoon is equal to 1/2 fluid ounce; this is about 14.8 milliliters. In Britain, traditional tablespoons varied somewhat in size, and various older references give sizes in the range from 1/2 to 5/8 Imperial fluid ounce (14.2-17.6 milliliters). Under the metric system the tablespoon has become more or less standardized at 15 milliliters in Britain, Canada, and New Zealand, 20 milliliters in Australia. The U.S. tablespoon equals 3 teaspoons or 1/16 cup; the traditional British tablespoonful was often equal to 4 teaspoonfuls or 1/10 teacupful. The metric tablespoon equals 3 teaspoons (4 in Australia). tablespoon or tablespoonful (tbsp, tblsp, or Tsp) [2] a unit of volume used in bartending. U.S. bartenders use a tablespoon of 3/8 fluid ounce or 1/4 jigger; this is equivalent to about 11.1 milliliters. tael or tahil a traditional unit of weight used throughout eastern Asia. During the colonial period, the tael was more or less standardized throughout the region at 4/3 ounce avoirdupois (1/16 catty, 1/12 pound, or about 37.8 grams). In Japan, however, the tael was identified with a slightly smaller traditional unit and is considered equal to 1.323 ounces (37.51 grams). The tael is usually considered equal to the Chinese liang. tan a traditional Chinese weight unit, now spelled dan in English transliteration. During the European colonial era the tan was equal to 100 cattys or 1600 taels. This is equivalent to 133.333 pounds, making the tan comparable to the European quintal as a commercial weight unit. In modern China the tan, or rather the dan, is equal to 100 jin, which is exactly 50 kilograms (110.231 pounds). teacupful a unit of liquid volume used in British food recipes. The teacupful is the same volume as an Imperial gill: 5 fluid ounces, 8.670 cubic inches, or about 137.7 milliliters. teaspoon or teaspoonful (tsp) [1] a unit of volume used in food recipes. The U.S. teaspoon is equal to 1/3 tablespoon or 1/48 cup; this is equivalent to 1/6 fluid ounce, about 0.30 cubic inches, or approximately 4.9 milliliters. In Britain, a traditional teaspoonful in the kitchen was equal to 1/8 Imperial fluid ounce or approximately 3.55 milliliters, but the medical teaspoonful was usually 5 milliliters. In metric kitchens in Britain, Canada, Australia, and New Zealand, a teaspoonful is exactly 5 milliliters. teaspoon or teaspoonful (tsp) [2] a unit of volume used in bartending. U.S. bartenders use a teaspoon equal to 1/8 fluid ounce or 1/12 jigger; this is equivalent to about 3.7 milliliters. technical atmosphere (at) a metric unit of pressure equal to one kilogram of force per square centimeter. The technical atmosphere equals about 980.665 millibars (mb), 98.0665 kilopascals (kPa), approximately 28.96 inches of mercury (in Hg), or 14.223 pounds of force per square inch (lb/in2). This is about 97% the average pressure of the earth's atmosphere at sea level. tebi- (Ti-) a binary prefix meaning 240 = 1 099 511 627 776. This prefix, adopted by the International Electrotechnical Commission in 1998, replaces tera- for binary applications in computer science. The prefix is a contraction of "terabinary." tera- (T-) a metric prefix meaning 1012, or one trillion. The prefix is derived from the Greek word for monster, teras. (Our words "terrible" and "terrific" have this same root.) tertian a traditional unit of volume for liquids. The tertian, like the tierce (see below) takes its name from the Latin for 1/3. It equals 1/3 tun, which is 2 tierces or 84 U.S. gallons (about 318 liters). tesla (T) the SI unit of flux density (or field intensity) for magnetic fields (also called the magnetic induction). The intensity of a magnetic field can be measured by placing a current-carrying conductor in the field. The magnetic field exerts a force on the conductor, a force which depends on the amount of the current and on the length of the conductor. One tesla is defined as the field intensity generating one newton of force per ampere of current per meter of conductor. Equivalently, one tesla represents a magnetic flux density of one weber per square meter of area. A field of one tesla is quite strong: the strongest fields available in laboratories are about 20 teslas, and the Earth's magnetic flux density, at its surface, is about 50 microteslas (µT). One tesla equals 10 000 gauss. The tesla, defined in 1958, honors the American electrical engineer Nikola Tesla (1856-1943), whose work in electromagnetic induction led to the first practical generators and motors using alternating current. tetrad [1] a unit of quantity equal to 4. tetrad [2] a unit of digital information equal to 4 bits or 1/2 byte. This unit, in various contexts, is also called a nibble, a quadbit, or a hexit. The unit seems to be more common in German, where it is spelled tetrade. TEU a unit of cargo capacity, especially for container ships. These ships carry cargo in standard metal boxes, called containers, which can be transferred easily to trains or trucks. TEU is an abbreviation for "twenty-foot equivalent unit." One TEU represents the cargo capacity of a standard container 20 feet long, 8 feet wide, and (usually) a little over 8 feet high, or half the capacity of a similar container 40 feet long. One TEU equals about 12 register tons (see ton [3] below) or 34 cubic meters. tex a unit used in the textile industry to measure the density of a single fiber of yarn. One tex equals a density of one gram per kilometer of length, or 1 mg/m. One tex equals 10 drex or 9 denier. therm (thm) a commercial unit of heat energy. The therm is equal to 100 000 Btu. Because there have been several definitions of the Btu, there are two official definitions of the therm. In the U.S., the legal definition (made in 1968) is that the therm equals 105.4804 megajoules. The European Union's definition, made in 1979 using the more current IT Btu, is 105.5060 megajoules. Either way the therm is equal to about 25 200 (large) calories or about 29.3 kilowatt hours of electrical energy. One therm can also be provided by about 96.7 cubic feet of natural gas. The therm has sometimes been confused with the thermie (next entry). The names of both units come from the Greek word for heat, therme. thermie (th) a metric unit of heat energy, part of the meter-tonne-second system sometimes used by European engineers. The thermie is equal to the amount of energy required to raise the temperature of 1 tonne of water by 1°C. The thermie is equivalent to 1000 (large) calories , 4.1868 megajoules or 3968.3 Btu. third [1] a unit used in music to describe the ratio in frequency between notes. Two notes differ by a minor third if the higher note has frequency exactly 6/5 times the frequency of the lower one, or by a major third if the higher note has frequency exactly 5/4 times the frequency of the lower one. On the standard 12-tone scale, the minor third is approximated by 3 half steps, corresponding to a frequency ratio of 21/4 = 1.1892; the major third is approximated by 4 half steps, corresponding to a frequency ratio of 21/3 = 1.2599. third ("') [2] a unit of time equal to 1/60 second. The symbol for this rarely-used unit is a triple prime, suggesting a natural progression from the minute (') through the second (") to the third ("'). thou [1] a British name for what Americans call a mil: a unit of distance equal to 0.001 inch. thou [2] an informal contraction of "thousand," sometimes used as a unit of quantity. thrave a traditional measure of grain in Scotland and northern England. A thrave is usually 24 sheaves, each about 30 inches (76 centimeters) in circumference. tick [1] an informal unit of time equal to the length of one cycle of a clock. A tick of a computer system's clock, also called a jiffy, is usually 0.01 second. A tick in athletics is the smallest increment of time measured in a timed competition, usually 0.1 second or 0.01 second. tick [2] an informal unit used in finance and investing to express the smallest measured change in a price or index. TID abbreviation for the Latin ter in die, three times a day, a unit of frequency traditionally used in medical prescriptions. tierce an old English unit of volume, equal to 1/3 butt or 42 United States gallons. The tierce is almost exactly 159 liters. The name of the unit is French; it is derived from the Latin tertius meaning 1/3. timber a traditional unit of quantity for furs equal to 2 score or 40. This unit, which persisted at least into the nineteenth century, originated because furs were shipped in bundles of 40 pressed between two boards or timbers. time zone an informal unit used to express differences in longitude between two places on the Earth. On the average, a time zone spans 15?of longitude. Of course, actual time zones have irregular boundaries, so this is only approximate. At the latitudes of the continental U.S., time zones average about 800 miles wide. tithe a traditional unit of proportion equal to 1/10. The word "tithe" is an old one, drawn directly from the Anglo-Saxon word for a tenth. TME a German abbreviation for Technische Mass Einheit (engineering mass unit). One TME is the mass accelerated at 1 m/s2 by a force of 1 kgf. One TME is equal to 9.80665 kilograms or 21.6200 pounds. This unit is sometimes called the metric slug in English, since the slug is defined in a similar way in the English system. to a traditional Japanese unit of volume. The to equals 10 sho, which is about 18.039 liters, 3.968 British imperial gallons, or 4.765 U.S. liquid gallons. tod a traditional unit of weight equal to 2 stone or 1 quarter. The tod is thus equivalent to 28 pounds or about 12.7 kilograms. "Tod" is an old German word meaning a load. toe a symbol for tonne of oil equivalent, a unit of energy used in the international energy industry. 1 toe represents the energy available from burning one tonne (metric ton) of oil; this is considered equivalent to approximately 7.4 barrels of oil, 1270 cubic meters of natural gas, or 2.3 tonnes of coal. 1 toe is equivalent to 41.868 gigajoules (GJ), 39.683 million Btu (MM Btu) or dekatherms, or 11.630 megawatt hours (MWh). tog a metric unit used to describe the insulating properties of cloth. If the flow of heat through the cloth is 1 watt per square meter, then the insulating value in togs is 10 times the temperature difference, in Celsius degrees (or kelvins), between the two sides of the cloth. This makes the tog equal to exactly 0.1 m2K/W or about 0.645 clo. "Togs" is a slang word meaning clothing, but its origin is not known. toise a traditional French unit of distance comparable to the British fathom. Like the fathom, the toise originally represented the distance between the fingertips of a man with outstretched arms. Introduced by Charlemagne in 790, the toise is such an ancient unit that toiser has become a verb meaning "to measure" or "to size up." The toise equals six pieds (French feet). Feet of different lengths were used in France, but based on the 18th century Paris pied the toise equals 6.395 (English) feet or 1.949 meters. This unit was widely used in the 19th century and hasn't died out entirely today. tola a traditional unit of weight in India and South Asia. Like the seer and the maund, the tola varied considerably from one area to another. The official size in British India was 180 grains, which is about 0.4114 ounce (avoirdupois) or 11.664 grams. There are 80 tolas in a seer, 3200 in a maund. tolerance unit a unit of relative distance used by engineers when they put round pegs in round holes. Whenever a shaft passes through a hole, a small distance (the tolerance) must be left so that the shaft will be able to turn. If the diameter of a shaft is D millimeters, one tolerance unit for that shaft is 0.001D + 0.45(cube root of D). The cube root term allows proportionately more tolerance for smaller shafts. The actual tolerance can then be stated as a multiple of the tolerance unit. ton (t or tn) [1] a traditional unit of weight equal to 20 hundredweight. In the United States, there are 100 pounds in the hundredweight and exactly 2000 pounds (907.185 kilograms) in the ton. In Britain, there are 112 pounds in the hundredweight and 2240 pounds (1016.047 kilograms) in the ton. To distinguish between the two units, the British ton is called the long ton and the American one is the short ton. In old England, a "tun" was a large cask used to store wine. Because these tuns were of standard size, more or less, the tun came to represent both a volume unit, indicating the capacity of a cask, and also a weight unit, indicating the weight of a cask when it was full. ton (t or tn) [2] a metric unit of mass, equal to 1000 kilograms, or approximately 2204.623 pounds avoirdupois. This metric ton is a bit smaller than the British long ton. The metric ton is now known officially as the tonne (see below). ton (t or tn) [3] a unit used traditionally to measure the cargo capacity of a merchant ship. During the Middle Ages, merchant ships were rated by the number of tuns of wine they could carry. Today the merchant marine ton is defined to be exactly 100 cubic feet, or approximately 2.8316 cubic meters. This unit is often called the register ton, since it is recorded in official registers of ships. ton (t or tn) [4] a unit of volume used traditionally to measure the "displacement" of ships, especially warships. One way to describe the size of a ship is to state the volume of sea water it displaces when it is afloat: in other words, the volume of that part of the ship below the waterline. The actual weight of sea water varies somewhat according to its temperature and how salty it is, but for this purpose it has been agreed that a long ton of sea water occupies about 35 cubic feet. Accordingly the displacement ton is defined to be exactly 35 cubic feet, or approximately 0.9911 cubic meter. Since this is a much smaller unit than the register ton, warships have a much higher "tonnage" than merchant ships. ton (t or tn) [5] a traditional unit of volume used for measuring the cargo of a ship, truck, train, or other freight carrier. This freight ton is exactly 40 cubic feet, or approximately 1.1326 cubic meters. ton (t or tn) [6] a unit of energy used for measuring the energy of an explosion, especially a nuclear explosion. In this usage, one ton is the amount of energy released by the explosion of one short ton of TNT. This is defined in the U.S. to equal exactly 4.184 gigajoules (GJ) or roughly 4 million Btu. ton (t or tn) [7] a unit of power used in refrigeration engineering. One ton of refrigeration is the power required to freeze one short ton of water at 0°C in 24 hours: this is 3.516 853 kilowatts or 4.7162 horsepower. A ton of refrigeration also represents an energy consumption rate of almost exactly 200 Btu/min or 0.8396 Cal/s. t?de a traditional Danish unit of volume, equal to 144 pots or about 139 liters. The t?de holds 30.6 British imperial gallons, making it comparable to the barrel. Similarly, one of the meanings of the German word Tonne is "barrel." tone another name for the step [2] as a musical unit describing the ratio in frequency between notes. tonelada a traditional weight unit of Spain and Portugal corresponding to the English and metric ton. The traditional Spanish tonelada equals 2000 libras or about 2028 pounds (919.9 kilograms). The Portuguese tonelada, however, equals 1728 Portuguese libras or arratels; this is about 1748.6 pounds or 793.15 kilograms. tonne (t) a metric unit of mass equal to 1000 kilograms or approximately 2204.623 pounds avoirdupois. The SI recommends this French spelling for the metric ton (see ton [2] above) to distinguish it clearly from the long and short tons of customary usage. The SI also recommends that large masses be stated as multiples of the tonne. Thus a mass of 106 kg = 109 g should be called 1 kilotonne (kt) instead of 1 gigagram. In the United States, the Department of Commerce recommends that this unit be called the metric ton. tonneau the traditional French ton, equal to 2000 livres or about 979 kilograms (1.079 U.S. ton). The tonneau was also used as a measure of volume equal to 42 cubic pieds (50.84 cubic feet, or about 1440 liters). In the wine trade, the tonneau was a shipment of 100 cases, or 1200 bottles (about 900 liters of wine). ton of force (tnf or tn) a traditional unit of force, equal to 2000 pounds of force (about 8.8964 kilonewtons) in the U.S. and 2240 pounds of force (about 9.9640 kilonewtons) in Britain. ton per square inch (tnf/in2 or tsi) a unit of pressure traditionally used in engineering. In the U.S., 1 tsi = 2000 pounds per square inch or about 13.790 megapascals (MPa). In Britain, 1 tsi = 2240 pounds per square inch or about 15.444 megapascals. Torino number an arbitrary scale adopted in 1999 to express the likelihood that an asteroid or comet might collide with the Earth causing damage. Named for Torino (Turin), Italy, site of a June 1999 conference on near-Earth objects, the scale is intended to convey accurately the appropriate level of concern caused by newly-discovered objects, thus discouraging sensational press reports. The scale ranges from (object certain to miss the Earth or too small to cause significant damage) to 10 (object certain to hit the Earth and large enough to cause catastrophic damage and global climate disruption). An official description of the scale is included. torr (Torr) a non-metric unit of pressure equal to exactly 1/760 atmosphere, about 1.333 22 millibars, or 133.322 pascals. The pressure of 1 atmosphere is almost exactly equivalent to the pressure of a column of 760 millimeters of mercury in a mercury barometer. As a result, 1 torr is the same thing as 1 mmHg within 1 part per million. In engineering, the pressures of near-vacuums are often stated in torr. The unit is named for Evangelista Torricelli (1608-1647), the Italian scientist who invented the barometer. tot a unit of volume for liquor. Generally the term is used informally, with no fixed definition. However, in British pubs the usual understanding is that a tot is 1/6 gill; this is equivalent to 5/6 Imperial fluid ounce or about 23.7 milliliters. tovar a traditional weight unit of Bulgaria, equal to 100 oka; this is roughly 280 pounds or 128 kilograms. township (twp) a traditional unit of area in the U. S., equal to 36 sections or the area of a square 6 miles on a side (about 93.24 square kilometers). tpi an abbreviation for "threads per inch," a unit commonly used to describe screws and other items with similar threads. traffic unit an alternate name for the erlang. triad a unit of quantity equal to 3. triennium a traditional unit of time equal to 3 years. trilliard a unit of quantity equal to 1021, which is one sextillion in American terminology or 1000 trillion in traditional British terminology. The name is coined to parallel milliard, which has long been a name for 1000 million. trimester a unit of time equal to 3 months or 1/4 year; another name for the quarter [2]. At certain U.S. colleges a trimester is an academic term roughly 14 weeks long. trio a traditional unit of quantity equal to 3. The word is Italian. It originated in music during the Baroque era (early 1700's) meaning a composition in three parts. troland (Td) a unit used in photometry to measure the amount of light reaching the retina of the eye. The "retinal illuminance" in trolands is equal to the luminance of the light shining on the eye, measured in candelas per square meter (cd/m2), multiplied by the area of the pupil open to the light, in square millimeters (mm2). This calculation makes the troland technically equal to the microcandela (µcd), a unit of luminous intensity, but retinal illuminance is a physical qunatity different from luminous intensity. The unit is named for the American experimental psychologist Leonard T. Troland (1889-?). truss a traditional weight unit, generally equal to 4 stone or 56 pounds (about 12.7 kilograms). The truss was used primarily for measuring hay. In 1795, Parliament specified that a truss of hay should equal 56 pounds for old hay or 60 pounds for new hay. tsubo an informal unit of area used in Japan to measure area inside buildings. One tsubo is an area of about 3.3 square meters (3.95 square yards), an area that can be covered neatly by two traditional tatami mats. t'sun a Chinese unit of distance equal to 0.1 ch'ih or about 1.41 inches (3.58 centimeters). tu a traditional Chinese unit of distance equal to 250 li. Using the late nineteenth century definition of the li as 2115 feet, the tu was equal to 100.14 miles or 161.13 kilometers. tumbler another name for the U.S. cup. tumblerful another name for the breakfast cup, a unit of volume used in British food recipes. tun a unit of volume used for wine and other liquids. "Tun" is an old French word for a large cask used in shipping wine. Tuns of various sizes were used throughout the Middle Ages. More recently the tun has been regarded as equal to 2 butts or 252 U. S. gallons; this is equivalent to 33.6875 cubic feet or about 953.93 liters. See ton [3] and tonneau, above. turbidity unit see NTU. turn another name for a revolution, that is, a unit of angle measure equal to 360? twain an old word for the number two, derived from the Anglo-Saxon twegen. The American author Samuel Clemens (1835-1910), who had been a riverboat pilot on the Mississippi in his youth, took his literary name from a traditional phrase "mark twain", meaning "exactly two" fathoms of water. This was the minimum depth needed for the boats to operate safely without running aground. typp abbreviation for thousand yards per pound, a unit formerly used to describe yarn density. 1 typp equals about 2.015 907 meters per gram, and n typp corresponds to a yarn density of 2.015 907/n tex (see above). U u the SI symbol for the unified atomic mass unit, defined in 1960 and accepted by both chemists and physicists. U [1] a commercial unit of thermal conductance (heat flow). The U factor, as it is also called, is the conductance through an insulator as measured in Btu's of energy conducted times inches of thickness per hour of time per square foot of area per ¡ÆF of temperature difference between the two sides of the material. The U factor is numerically equal to 1 divided by the R value. U [2] a unit of distance used to measure the height of audio, video, or computer equipment components and the height of the standard racks in which these components are mounted. 1U is equal to 1.75 inches (44.45 millimeters), so that, for example, a 2U component is 3.5 inches high, and a 22U rack houses a stack of components 38.5 inches high. U [3] usual symbol for the enzyme unit. ua a symbol for the astronomical unit. unit when counting, the word "unit" means "one." For example, if a car dealer expects a shipment of 20 units, that means 20 cars. In addition, the word is used as a shorthand for a variety of named "units," such as the international unit in pharmacology. unit (of blood) a unit of volume for human blood and various blood components or products. A unit of whole blood is 450 milliliters, which is about 0.9510 U.S. pint. For components of blood, one unit is the amount of that substance that would normally be found in one unit of whole blood. The adult human body contains roughly 12 units of whole blood. unit (magnetic) pole a CGS unit measuring the strength of a magnetic pole. A unit magnetic pole repels an identical pole at a distance of one centimeter in a vacuum with a force of one dyne. The unit magnetic pole equals about 125.6637 nanowebers (nWb). universal time (UT or Z) the correct name for the time system previously called Greenwich Mean Time (GMT): the standard time at longitude 0¡Æ. Universal time is five hours later than Eastern Standard Time in the U.S. It is always stated on the 24-hour clock; thus an event which occurs at 1:26:15 pm Eastern Standard Time occurs at 18:26:15 UT (five hours after 13:26:15). Technically, the time shown on clocks is figured from Coordinated Universal Time (UTC), an international system of time measurement regulated by very precise atomic clocks. From time to time (either on June 30 or December 31), UTC is adjusted by the addition of a "leap second". This event, widely reported in the press but poorly understood by the public, keeps UTC within 0.9 seconds of mean solar time at longitude 0¡Æ as measured by the Earth's slightly uneven rotation. unze a traditional German weight unit, corresponding to the English ounce. The unze equals 1/16 pfund. Although the pfund has been assimilated into the metric system (as 500 grams), the unze is effectively obsolete. It varied in size from about 28 to 35 grams. The word comes directly from the Latin uncia; the plural is unzen. USP unit a unit used in the United States to measure the mass of a vitamin or drug based on its expected biological effects. For each substance to which this unit applies, the U. S. Food and Drug Administration has determined the biological effect associated with a dose of 1 USP unit. Other quantities of the substance can then be expressed in terms of this standard unit. In most cases, the USP unit is equal to the international unit (IU). "USP" is an abbreviation for the United States Pharmacopeia, a handbook describing the established properties of drugs legal for use in American medicine. V var a unit of the reactive electric power delivered by an alternating current (AC) circuit. In an AC circuit, the electric potential or voltage (measured in volts) and the current (in amperes) alternate direction, varying smoothly according to sine curves. If there were no resistance in the circuit, the variations of the current would lag the variations in the voltage by 1/4 cycle, or 90¡Æ. In real circuits, the current can be separated into two parts: the "reactive" part, which lags the voltage by 90¡Æ, and a part in phase with the voltage. The reactive part does no useful work; it simply heats the conductor.The reactive power is the product of the voltage and the reactive part of the current. The name of the unit is an acronym for volt-ampere-reactive. vara a traditional unit of distance in Spanish- and Portuguese-speaking countries. The length of the vara varied (no pun intended), but in Spanish Latin America it was generally about 33 inches or a little longer. In Texas, where it was often used in land measurement, the vara was defined to equal exactly 33 1/3 inches, which is equivalent to 84.667 centimeters. In California, the vara was considered equal to 33 inches (83.82 centimeters) and in Mexico the former standard was 32.993 inches (83.802 centimeters). The Spanish vara is shorter; it equals 32.908 inches or 83.587 centimeters. The Portuguese vara, on the other hand, is much longer; it equals 5 palmos or about 110 centimeters (43.3 inches). The word vara means a stick or pole. vedro a traditional Russian unit of volume equal to 100 charki. The vedro is about 12.30 liters (3.249 U.S. liquid gallons or 2.706 British imperial gallons). The word vedro means a bucket. verchok a traditional Russian unit of distance equal to 1/16 arshin, 1.75 inches or 4.445 centimeters. The plural is verchoki. verge an old name for the yard, taken from the Latin word virga for a twig or stick. verst a traditional Russian unit of distance, formerly used throughout eastern Europe. The verst equals 1500 arshin, which is 3500 feet, 0.662 88 mile, or 1066.8 meters. The unit is sometimes known by its German name, werst. In Finland it is called the virsta. viertel a traditional unit of volume in several European countries. Oddly, although the name means "quarter" in German the viertel is not really 1/4 of any other unit. The Danish viertel equals 8 pots or about 7.74 liters (2.04 U.S. liquid gallons or 1.70 British imperial gallons). In Switzerland the viertel is 40 schoppen, which is exactly 15 liters (3.9626 U.S. liquid gallons or 3.3000 British imperial gallons). virgate an old English unit of land area equal to 1/4 hide. This was roughly 30 acres or 12 hectares. vision a term often used with a Snellen fraction in phrases such as "20/20 vision." volt (V) the SI unit of electric potential. Separating electric charges creates potential energy, which can be measured in energy units such as joules. Electric potential is defined as the amount of potential energy present per unit of charge. Electric potential is measured in volts, with one volt representing a potential of one joule per coulomb of charge. The name of the unit honors the Italian scientist Count Alessandro Volta (1745-1827), the inventor of the first battery. volt ampere (V¡¤A) a unit of electrical load used in power engineering. In an alternating current circuit, if the potential (measured in volts) and the current (measured in amperes) vary in phase with each other, then the power delivered (measured in watts) is the product of the potential and the current. In actual circuits, the potential and voltage are usually out of phase (see var, above), causing the device receiving the power to draw more current than its wattage requirements would suggest. The product of the potential and the actual current is the load, in volt amperes. volume unit (vu) a unit used in telecommunications to describe the volume of a radio or television signal carrying complex information such as speech or music. These waves vary wildly in amplitude in order to represent the message they carry, so it is difficult to describe mathematically how large their "average" amplitude is. To solve this problem, the American Standards Association decided in 1951 to establish as a reference a signal which generates 1 milliwatt of power in a circuit having an impedance of 600 ohms. The volume of an incoming signal, in volume units, is equal to the number of decibels by which it exceeds this reference level. Thus the volume unit is (in a complicated way) a variation of the decibel. v/v an abbreviation for "by volume," used in chemistry and pharmacology to describe the concentration of a substance in a mixture or solution. Thus 2% v/v means that the volume of the substance is 2% of the total volume of the solution or mixture. See also w/v. W watch a traditional unit of time, defined as the time a sentry stands watch or a ship's crew is on duty. On both land and sea, one watch is usually equal to 4 hours. At sea, the evening watch (16-20 hours, or 4-8 pm) is often divided into two shorter watches called "dog watches." When dog watches are in effect, sailors will have watch assignments that rotate through the day instead of falling at the same hours every day. Watches at sea are divided into 8 bells (4 bells for dog watches). water column (WC) more properly inch of water column, a traditional unit of pressure used in plumbing. water inch a traditional unit of water flow, supposed to equal the flow through an opening one inch in diameter, assuming the water just covers the opening so the flow is caused only by gravity. There are differing estimates of this flow rate. One estimate is 14 pints per minute or 2520 gallons per day (this is equivalent to 6.530 liters per minute). Another is 500 cubic feet per day, which is much larger: 3740 gallons per day or about 9.832 liters per minute. See also miner's inch. watt (W) the SI unit of power. Power is the rate at which work is done, or (equivalently) the rate at which energy is expended. One watt is equal to a power rate of one joule of work per second of time. This unit is used both in mechanics and in electricity, so it links the mechanical and electrical units to one another. In mechanical terms, one watt equals about 0.001 341 02 horsepower (and thus one kilowatt equals about 1.341 02 horsepower). In electrical terms, one watt is the power produced by a current of one ampere flowing through an electric potential of one volt. The name of the unit honors James Watt (1736-1819), the British engineer who built the first practical steam engines. watt hour (W¡¤h) a common metric unit of work or energy, representing the energy delivered at a rate of one watt for a period of one hour. This is equivalent to exactly 3.6 kilojoules (kJ) of energy, or about 3.412 141 Btu, 0.859 846 (kilogram) Calories, or about 2655 foot pounds. wave or wavelength a unit of relative distance equal to the length of a wave: this could be a light wave, a radio wave, or even an ordinary water wave. In communications engineering, the length of an antenna is often stated in waves, and in optics, the surfaces of lenses and mirrors are required to be precisely polished to within a very small fraction of a wave. weber (Wb) the SI unit of magnetic flux. "Flux" is the rate (per unit of time) in which something crosses a surface perpendicular to the flow. If the something is a magnetic field, then the magnetic flux across a perpendicular surface is the product of the magnetic flux density, in teslas, and the surface area, in square meters. If a varying magnetic field passes perpendicularly through a circular loop of conducting material, the variation in the field induces a electric potential in the loop. If the flux is changing at a uniform rate of one weber per second, the induced potential is one volt. This means that numerically the flux in webers is equal to the potential, in volts, that would be created by collapsing the field uniformly to zero in one second. One weber is the flux induced in this way by a current varying at the uniform rate of one ampere per second. The weber is a large unit, equal to 108 maxwells, and practical fluxes are usually fractions of one weber. (Because of this, when we want to induce an electric potential in a conductor with a changing field, as we do in all electric generators, transformers and electric motors, we loop the conductor into hundreds of coils, thus adding together the small voltages induced in each loop by the changing field.) The unit honors the German physicist Wilhelm Eduard Weber (1804-1891), one of the early researchers of magnetism. week (wk) a traditional unit of time equal to seven days. The custom of the seven-day week, with one day set aside for rest and religious observance, goes back thousands of years to the ancient civilizations of the Middle East. Christians inherited the seven-day cycle from the Jewish religion. When the Roman Emporer Constantine legalized Christianity, early in the fourth century CE, the week was added to the Roman calendar. Since none of the units of Roman date-keeping (the month, the quarter, and the year) equal a whole number of weeks, this made it necessary for the first time to have tables (we call them calendars!) showing the ever-changing relationship between the days of the week and the dates of the month. Link: The World Calendar Association promotes efforts to reform the calendar so that weeks and months would have a fixed relationship. Note: There are different traditions as to which day of the week is the first. In western countries, most calendars show Sunday as the first day of the week, but the International Standards Organization (ISO) specifies that the week begins with Monday. There are also different ideas about how to number the weeks of the year, which is sometimes necessary for business purposes. The official solution to this question is that week 1 of the year is the week (beginning with Monday) that contains January 4. Since 1999 January 4 falls on Monday, week 1 of 1999 is the week January 4-10. Week 1 of 2000 is the week January 3-9. Link: ISO 8601 from Markus Kuhn. werst a German spelling, sometimes seen in English as well, for a Russian distance unit, the verst. wey a historic English unit. The word comes from the old English w©¡ge, meaning weight, and originally the wey was a weight unit representing about two hundredweight. Later it came to be used as a volume unit for a variety of dry commodities. Its size varied. Roughly speaking, the wey represented about 40 bushels, 2 cubic yards, or 1.5 cubic meters. wind chill index (WCI) a measurement of the combined cooling effect of low air temperature and wind on the human body. The index was first defined by the American Antarctic explorer Paul Siple in 1939. As currently used by U.S. meteorologists, the wind chill index is computed from the temperature T (in ¡ÆF) and wind speed V (in mi/hr) using the formula: WCI = 0.0817(3.71 sqrt(V) + 5.81 - 0.25V)(T - 91.4) + 91.4. The metric equivalent, for T in ¡ÆC and V in km/hr, is: WCI = 0.045(5.27 sqrt(V) + 10.45 - 0.28V)(T - 33) + 33. For very low wind speeds, below 4 mi/hr or 6 km/hr, the WCI is actually higher than the air temperature, but for higher wind speeds it is lower than the air temperature. wine gallon a former English unit of volume equal to 231 cubic inches. The wine gallon was adopted as the official gallon for liquid measurement in the United States, so now it is usually called the U.S. gallon. wineglass an informal unit of volume used in U.S. bartending, equal to 4 fluid ounces or about 118.3 milliliters. This unit is the same as the traditional gill. wineglassful a unit of volume used in British food recipes. The wineglass holds 2.5 fluid ounces, 5 tablespoonfuls, 1/2 gill, or about 142 milliliters. One wineglassful is equal to 0.60 U.S. cup. word a unit of information in computer science, representing the amount of data processed by a computer in a single instruction. The size of a word can vary with the computer system, but often one word is equal to 2 bytes or 16 bits. w/v an abbreviation for "weight by volume," a slightly confusing phrase used in chemistry and pharmacology to describe the concentration of a substance in a mixture or solution. The weight by volume is the mass (in grams) of the substance dissolved in or mixed with 100 milliliters of solution or mixture. For example, the concentration of fluoride in toothpaste is usually about 0.15% w/v, meaning that there is 0.15 gram of fluoride per 100 milliliters of toothpaste. Thus 1% w/v is equal to 1 gram per deciliter (g/dL) or 10 grams per liter. w/w an abbreviation for "by weight," used in chemistry and pharmacology to describe the concentration of a substance in a mixture or solution. Properly speaking, 2% w/w means that the mass of the substance is 2% of the total mass of the solution or mixture. X x or X the usual symbol for power as a unit of magnification. More generally, x is used with its mathematical meaning, "times," to indicate that a measurement is a multiple of some standard or reference measurement. For example, an image marked 200x is shown at 200 times actual size, and a 10x CD-ROM drive is one capable of spinning 10 times faster than the "normal" or standard speed. X unit (Xu) a unit of distance formerly used for measuring the wavelength of X rays and gamma rays. The X unit is approximately 1.0002 x10-13 meter, 0.001 002 angstrom, or 100.2 femtometers. The wavelength of these powerful forms of radiation is now measured in picometers (pm) or femtometers (fm). The unit was defined by the Swedish physicist K. M. G. Siegbahn in 1925, at a time when the wavelengths could not be measured directly. The definition was made in terms of the spacing between planes of the calcite crystals used in the measuring apparatus. Siegbahn, who received the Nobel Prize in Physics in 1924 for his work in X-ray spectroscopy, aimed to define a unit equal to approximately 100 fm, and he succeeded admirably. Y yard (yd) [1] a traditional unit of distance equal to 3 feet or 36 inches. The yard is the fundamental unit of distance in the English customary system. The word comes from a Saxon word gird or giard meaning a stick, and the Saxon kings are believed to have kept official yardsticks at Winchester as early as the middle of the 10th century. As for the length of the yard, there is a traditional tale that Henry I decreed in the 12th century that the yard should be the distance from the tip of his nose to the tip of his outstretched finger, thus defining the yard as exactly 1/2 fathom. By the late 13th century it appears that the yard was within a millimeter of its present length. Today one yard is officially equal to exactly 91.44 centimeters or 0.9144 meter; this definition was established in Britain by the Weights and Measures Act of 1963 and has now been accepted in the United States as well. yard (yd) [2] the yard is traditionally used as a unit of area for materials sold in standard rolls, such as cloth, carpet, linoleum, fencing, and so on. In each case, one yard represents an area one yard long and as wide as the roll width. See bolt. yard (yd) [3] a cubic yard (about 764.555 liters). Bulk commodities like sand or topsoil are often sold by the "yard." yard of ale a traditional Scottish measure of volume. A yard of ale is roughly 2.5 pints (1.4 liters) served in a slender, yard-tall glass. year (a or y or yr) [1] a unit of time, defined to be the period of time required for the Earth to make one revolution around the Sun. To be more precise, the year we use in ordinary life is the interval between two arrivals of the Sun at the Tropic of Capricorn, marking the winter solstice. Astronomers call this unit the tropical year. There are 365.242 199 days in a tropical year, or, to be even more precise, 31 556 925.9747 seconds. The official abbreviation for the year in scientific writing is a, taken from the French word for year, annee, or the Latin word, annum. Thus 1 Ma stands for a million years and 1 Ga for a billion years. year (a or y or yr) [2] a traditional unit of time usually equal to 365 or 366 days. We need a whole number of days for the calendar year used in ordinary life. Ancient astronomers knew that the year is approximately 365 days long, and we now know the correct figure is about 365.2422 days. If we use 365 as the number of days in every calendar year, the extra 0.2422 day adds up quickly and causes large errors in predicting the seasons. To solve this problem, the Roman emperor Julius Caesar decreed in 46 BC that the calendar year should have 365 days generally, but that every fourth year should have an extra, or 366th, day. The longer year is called a leap year. In this Julian calendar, four years equal exactly 1461 days, so the average Julian year is exactly 365.25 days. This was a big step toward accuracy in the calendar, but the Julian year is too long by 0.0078 day, or about 11 minutes. By the time of the Renaissance, these 11-minute errors had accumulated to a total error of about 10 days, so that the spring equinox was occurring near March 11 instead of March 21. In 1582, Pope Gregory XIII decreed that 10 days should be dropped from the calendar: the day after 1582 October 4 was October 15. To reduce future errors, the pope further decreed that years divisible by 100 are not leap years unless they are also divisible by 400. Thus 2000 and 2400 are leap years, but 2100, 2200, and 2300 are not. With this adjustment, there are exactly 146 097 days in every 400 years, and the average Gregorian year is exactly 365.2425 days. The Gregorian year is still too long, but only by 0.000 301 2 day, or about 26 seconds. It takes more than 3300 years for this error to accumulate to 1 day, so the calendar year and the tropical year are in good enough agreement to last us a long time. yocto- (y-) a metric prefix denoting 10-24 (one septillionth). Adopted by the CGPM in 1990, the prefix is derived from the Latin octo and Greek okto, meaning 8, because this is the eighth prefix (n = 8 in 10-3n) in the SI system of metric prefixes. The y was added arbitrarily to provide a non-confusing letter for abbreviations. yoke another name for a pair. The yoke is used in describing teams of animals, especially oxen, used to pull plows or wagons. yotta- (Y-) a metric prefix denoting 1024 (one septillion). The prefix was coined to parallel the prefix yocto-. Z Z a common symbol for universal time. Z is the first letter of the German word for time, Zeit. This symbol is often used with a 4-digit statement of the time in hours and minutes; thus 8:43 UT is written 0843 Z. The symbol is often pronounced "zulu," the name of the letter Z in the international radio alphabet. z symbol for redshift, a unit of relative distance used in astronomy. zak a Dutch unit of volume, now interpreted as a metric unit equal to the hectoliter (100 liters). The hectoliter is equivalent to 26.417 U.S. gallons, 21.999 British imperial gallons, or 3.5315 cubic feet. This is a metric version of the British sack. zentner (Ztr) a traditional German unit of weight or mass, comparable to the traditional quintal and to the English hundredweight. Similar units were used in Scandinavia. Originally the zentner was equal to 100 pfunde, or German pounds. 100 pfunde varied from 110 to as much as 120 pounds avoirdupois, depending on the market. After the introduction of the metric system in Germany, the zentner was redefined to equal to exactly 50 kilograms, which is about 110.231 pounds. In English, the zentner is commonly spelled centner. zepto- (z-) a metric prefix denoting 10-21 (one sextillionth). Adopted by the CGPM in 1990, the prefix is derived from the Latin septem, meaning 7, because this is the seventh prefix (n = 7 in 10-3n) in the SI system of metric prefixes. The s was replaced by a z to avoid confusion with the abbreviation for the second, which is a fundamental SI unit. zetta- (Z-) a metric prefix denoting 1021 (one sextillion). The prefix was coined to parallel the prefix zepto-. Zhubov scale a scale for reporting ice coverage of polar seas; see ball. zoll the traditional German inch, equal to 1/12 fuss. Originally the zoll was equal to 1.037 inches (about 2.634 centimeters). In Switzerland, it is now considered a metric unit equal to exactly 3 centimeters (1.1811 inches). The plural is zollen. | |
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