acre (ac or A)
An old unit of area used for measuring the area of land . The acre (old English word meaning field) was originally defined as the area that could be plowed in a day by a yoke of oxen. The acre was defined as the area of a field one furlong long by 1/10 furlong wide. In metric the unit corresponding to the acre is the hectare, which is 10,000 square metre. One acre is equal to 0.404 687 3 hectare.

ampere (A or amp) The SI base unit of electric current, named after the French physicist Andr-Marie Ampre (1775-1836). The ampere is defined as the constant current which, if maintained in two straight parallel conductors of infinite length, of negligible circular cross-section, and placed 1 metre apart in vacuum, would produce between these conductors a force equal to 2 x 10-7 newton per metre of length.

ampere hour (Ah or amp hr )
A 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 exactly 3600 coulomb.

ampere per meter (A/m)
The SI derived unit of magnetic field strength.

angstrom ()
A metric unit of length, equal to 0.1 nanometre or 10-10 metre. Angstrom unit named after the Swedish physicist Anders Jon ngstrm (1814-1874) and is used most often to measure extremely small lengths like the wave length of light.

arcminute (' , min)
A unit of angle measurement, also called the minute of arc, equal to 60 arcseconds and to 1/60 degree. There are 21 600 arcminutes in a circle. Care is needed not to confuse symbols with units of length or time. The SI recommends ' as the symbol for arcminute.

arcsecond (",sec, s, as)
A unit of angle measure, also called the second of arc, equal to 1/60 arcminute. One arcsecond is a very small angle: there are 1 296 000 seconds in a circle. Care is needed not to confuse symbols with units of length or time. The SI defines s as the symbol for the time unit and recommends " as the symbol for the arcsecond.

are (a)
An old unit of area equal to 100 square metre.

astronomical unit (ua or au 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 centre of the Earth to the centre of the Sun. The currently accepted value is 1.495 978 x 1011 metre or about 92 955 807 miles. The astronomical unit is a convenient yardstick for measuring the distances between objects in the Solar System. This unit is accepted for use with SI units.

atmosphere (atm)
A unit of pressure equal the average pressure of the Earth's atmosphere at sea level. One atmosphere is equal to 1.013 25 bar or 1.013 25 x 105 Pa or 760 mmHg.

atomic mass unit (u)
The unit of mass used for measuring the masses of atoms and molecules. Originally these relative masses were based on hydrogen, known to be the lightest element, having a mass of 1 u, and all the other atoms should have masses which are whole-number multiples of this (then unknown) mass of the hydrogen atom. Since 1960 the unified atomic mass unit has been defined as 1/12 the mass of the carbon-12 atom. 1u = 1.66 x 10-27 kg.

atomic number
A unit of measurement, equal to the number of electrons surrounding a neutral (uncharged) atom, and also to the number of protons in the nucleus. The atomic number was originally defined simply as an index describing the position of an element in the periodic table.

Avogadro constant (NA), Avogadro's number
A unit of relative quantity equal to the number of atoms or molecules per mole of a substance. The currently accepted value is 6.022 1415 x 1023 per mole. The atomic mass unit in grams, is equal to one divided by this number. The unit is named after the Italian chemist and physicist Amadeo Avogadro (1776-1856). Avogadro was the first to conclude from Dalton's atomic theory that equal volumes of gases (at the same temperature and pressure) must contain equal number of molecules.

bar (bar)
A unit of pressure, equal to 105 pascal. One bar is roughly the same as the average pressure of the Earth's atmosphere (atm), which is 1.013 25 bar. A barometer an instrument for measuring barometric pressure of the atmosphere, usually in units of millibar (mbar) or as the height in millimetres, of a column of mercury (mmHg).

barn (b)
A unit of area used in nuclear physics. One barn is equal to 10-28 square metre or 100 square femtometre.

becquerel (Bq)
The SI derived unit of activity, usually meaning radioactivity. One becquerel is the radiation caused by one disintegration per second. The unit is named after the French physicist, Antoine-Henri Becquerel (1852-1908), the discoverer of radioactivity. Note: both the becquerel and the hertz are basically defined as one event per second, yet they measure different things.

bel (B)
A logarithmic measure of sound intensity, invented by engineers of the Bell telephone network in 1923 and named after the inventor of the telephone, Alexander Graham Bell (1847-1922). The scale is logarithmic; if the difference in sound intensity is 1 bel the difference is 10 times and 2 bels corresponds to an increase of 10 x 10 or 100 times in intensity. The beginning of the scale, 0 bels, was originally intended to represent the faintest sound that people can hear. In practice, sound intensity is almost always stated in decibels. One bel is equal to approximately 1.151 293 nepers.

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 picometre (pm), or 52.918 x 10-12 metre. The unit was named after the Danish physicist Niels Bohr (1885-1962).

boltzmann's constant (k)
The number that relates the average energy of a molecule to its absolute temperature. Boltzmann's constant is approximately 1.38 10-23 J/K.

British thermal unit (Btu or 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. Like the calorie below Btu can have slightly different values, so for accurate work it is necessary to specify which is being used.

  • BtuIT = 1055.056 J

  • Btuth = 1054.350 J

  • Btumean = 1055.87 J

  • 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. There can be some confusion in some of the units of energy. The calorie can take 5 different values and, while these do not vary by very much, for accurate work it is necessary to specify which calorie is being used. The 5 calories are known as the:

  • International Table calorie = calIT = 4.1868 J

  • thermochemical calorie = calth = 4.184 J

  • mean calorie = calmean = 4.190 02 J

  • 15 degree C calorie = cal15C = 4.185 80 J

  • 20 degree C calorie = cal20C = 4.181 90 J.

  • As a further complication, in working with food and expressing nutritional values, the unit of a Calorie (capital C) is often used to represent 1000 calories (kcal), and again it is necessary to specify which calorie is being used.

    candela (cd)
    The SI base unit for measuring the luminous intensity of light. Candela is the Latin word for "candle." The candela is defined as the luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency 540 x 1012 hertz and that has a radiant intensity in that direction of 1/683 watt per steradian.

    coulomb (C)
    The SI derived unit of electric charge. One coulomb is the amount of charge accumulated in one second by a current of one ampere (A s). The coulomb was named after the French physicist, Charles-Augustin de Coulomb (1736-1806).

    cubic unit
    The word cubic place before a unit name of length to indicate a unit of volume.

    cubic metre (m3)
    The SI derived unit of volume.

    curie (Ci)
    A unit of radioactivity. Originally, defined as the radioactivity of one gram of pure radium. Now the curie is defined exactly as 3.7 x 1010 atomic disintegration's per second, or becquerel (Bq), this being the best estimate of the activity of a gram of radium. The unit is named for Pierre and Marie Curie, the discoverers of radium.

    degree Celsius (C)
    A unit of temperature. The Celsius temperature scale was named after the Swedish astronomer and physicist Anders Celsius (1701-1744). The freezing point of water (at one atmosphere of pressure) was originally defined to be 0 C, while the boiling point is 100 C. Thus the Celsius degree is 1/100 of the difference between these two temperatures. In the SI system, the degree Celsius a derived unit from the base unit kelvin and is defined 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 C (0 K) 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 this is equivalent to the original definition. See also degree Fahrenheit, degree Rankine, kelvin,

    degree centigrade (C)
    Old name for the degree Celsius. The Celsius scale was called "centigrade" because it has 100 (centi-) gradations between the freezing point and boiling point of water.

    degree ( or deg)
    A unit of angle measurement, equal to 1/360 circle, 60 minutes, 3600 seconds, or about 0.017 453 293 radian.

    diopter (dpt or D)
    A metric unit used in optics to measure the refractive power of a lens. Each converging lens has a focal length, defined to be the distance from the centre of the lens to the point at which the lens focuses light. (Diverging lenses, which can be used to convert focused light to parallel rays, have a negative focal length.) 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.

    dyne (dyn)
    The CGS unit of force. One dyne is the force that accelerates a mass of one gram at the rate of one centimetre per second per second. Expressed in SI units, the dyne equals 10-5 newton.

    electronvolt (eV)
    A unit of energy used in physics. One electronvolt 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 is 1.602 176 53 x 10-19 joule. This unit is accepted for use with SI units.

    electronvolt-kilogram relationship ((1 eV)/c2)
    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 electronvolt is therefore equivalent to a mass of about 1.782 661 81 x 10-36 kg.

    electron mass (me)
    The mass of the electron, often used as a unit of mass in particle physics. An electron has a mass of about 9.109 3826(16) x 10-31 kg (equivalent to 5.109 989 x 105 eV).

    erg (erg)
    The unit of energy in the CGS system, equal to the work done by a force of one dyne acting through a distance of one centimetre. 1 erg = 10-7 J.

    degree Fahrenheit (F)
    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). Fahrenheit set 0 at the coldest temperature he could conveniently achieve using an ice and salt mixture, and he intended to set 96 as the temperature of the human body. The scale was later precisely defined by the freezing (-32 F) and boiling (212 F) points of water. 1F equals 5/9 C, but in converting between scales adjustment is needed for the zero points as well. To convert a temperature in F to C, first subtract 32 and then multiply by 5/9. In the other direction, to convert a temperature in C to F, first multiply by 9/5 and then add 32.

    farad (F)
    The SI derived unit of electric capacitance. A pair of conductors separated by an insulator can store a much larger charge than an isolated conductor. 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 or nanofarads. The unit was named after the British physicist Michael Faraday (1791-1867).

    faraday (Fd)
    A unit of electric charge. 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. This amount of charge, equal to about 96 485 coulombs, 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.

    foot (ft or ')
    a traditional unit of distance. Almost every culture has used the human foot as a unit of measurement. The International foot is equal to 0.3048 metre.

    g
    A symbol for the average acceleration produced by gravity at the Earth's surface (sea level). The actual acceleration of gravity varies depending on latitude, altitude, and local geology. The symbol g is often used informally as a unit of acceleration. The standard acceleration of gravity gn is defined to be exactly 9.806 65 meters per second per second (m/s). The name grav is also used for this unit. Note that g is also the symbol for the gram.

    gal (Gal)
    The CGS unit of acceleration. One gal is an acceleration of 1 centimetre per second per second (cm/s2). 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. 1 Gal = 1 cm s-2 = 10-2 m s-2.

    gallon (gal)
    The U.K. use a larger gallon than either of the U.S. gallons. The imperial gallon, designed to contain exactly 10 pounds of distilled water under precisely defined conditions, holds exactly 4.546 09 litre or cubic decimetre. The imperial gallon equals 1.20095 U.S. liquid gallons (British wine gallons) or 1.03206 U.S. dry gallons (British corn gallons).

    gamma (γ)
    A unit of magnetic flux density equal to 10-9 tesla (1 nanotesla) or 10-5 oersted (10 Oe). In geophysics, small changes in the Earth's magnetic field are traditionally stated in gammas. The nanotesla (nT) is now recommended for these measurements.

    gauss (G)
    The CGS unit of magnetic flux density. A field of one gauss exerts a force, on a conductor, placed in the field of 0.1 dyne per ampere of current per centimetre of conductor. One gauss represents a magnetic flux of one maxwell per square centimetre of cross-section perpendicular to the field. In SI units, one gauss equals 10-4 tesla. The unit was named after Karl Friedrich Gauss (1777-1855).

    grad or grade (g or gr or grd)
    A unit of angle measurement equal to 1/400 circle, 0.01 right angle, 0.9, or 54'. This unit was introduced in France, where it is called the grade, in the early years of the metric system.

    gram (g)
    A unit of mass in the metric system. The gram was originally defined to be the mass of one cubic centimetre of pure water, but is now defined to be 1/1000 of the mass of the standard kilogram. The kilogram, rather than the gram, is considered the base unit of mass in the SI.

    gravitional constant (G)
    A constant, G, in the mathematical formula of Newton's definition of gravitational force, F=Gm1m2/r2. G = 6.6742(10) x 10-11 m3 kg-1 s-2 or N m2 kg-2.

    gray (Gy)
    The SI derived unit of absorbed dose. Radiation carries energy, and when it is absorbed by matter the matter receives this energy. The absorbed 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 was named after the British physician L. Harold Gray (1905-1965).

    hartree (Eh)
    A unit of energy used in nuclear physics, equal to about 4.3598 x 10-18 joule or 27.212 electron volts. The unit was named after the British physicist and mathematician Douglas R. Hartree (1897-1958).

    hectare (ha)
    A metric unit of land area, equal to 100 are. One hectare is a square hectometre (10 000 m2) or 2.471 054 acre.

    henry (H)
    The SI derived unit of electric inductance. A changing magnetic field induces an electric current in a conductor 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 it is the current which changes, then the induced field will produce a potential difference within the coil: if the inductance is one henry a current change of one ampere per second produces a potential difference of one volt. The unit was named after the American physicist Joseph Henry (1797-1878).

    hertz (Hz)
    The SI derived 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 unit was named after the German physicist Heinrich Rudolf Hertz (1857-1894). The becquerel, also equal to one "event" per second, is used to measure the rates of things which happen randomly or unpredictably.

    horsepower (hp)
    An old unit of power originating from power exerted by a horse. The horsepower was defined by James Watt (1736-1819) who determined that a horse is typically capable of a power rate of 550 foot-pounds per second. Today the SI unit of power is named for Watt, and one horsepower is equal to approximately 746 watts. (Slightly different values have been used in certain industries.)

    hour (h or hr)
    A traditional unit of time, equal to 60 minutes, or 3600 seconds, or 1/24 day.

    Imperial units
    The units of the British Imperial system, adopted in 1828. The basic units of the system are the foot, the avoirdupois pound, and the imperial pint.

    inch (in or ")
    A traditional unit of distance equal to 1/12 foot or exactly 2.54 centimetre.

    jansky (Jy)
    A unit used in radio astronomy to measure the flux density of radio signals from space. One jansky equals a flux of 10-26 watts per square metre of receiving area per hertz of frequency band (W m-2 Hz-1). Although it is not an SI unit, the jansky is widely used by astronomers. Named after 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.

    joule (J)
    The SI derived unit of energy. Energy is said to exist in a variety of forms, each of which corresponds to a separate energy equation, but all resulting with the same energy unit of the joule. Some of the more common forms of energy are: kinetic energy, heat, potential energy, chemical energy, electrical energy, electromagnetic radiation, matter and antimatter. One joule is defined as the amount of work or energy exerted when a force of one newton is applied over a displacement of one metre. One joule is the equivalent energy of one watt of power radiated or dissipated for one second. The joule was named after the British physicist James Prescott Joule (1818-1889).

    kelvin (K)
    The SI base unit of temperature is 1/273.16 of the thermodynamic temperature of the triple point of water (temperature where water exists as gas, liquid and solid simultaneously). The unit kelvin and its symbol K should be used to express both thermodynamic temperature and an interval or a difference of temperature. In addition to the thermodynamic temperature (symbol T) there is also the Celsius (symbol t) defined by the equation t=T-T0 where T0=273.15 K. Celsius temperature is expressed in degree Celsius (symbol C). The unit 'degree Celsius' is equal to the unit 'kelvin', and a temperature interval or a difference of temperature may also be expressed in degrees Celsius. (The word degree and the sign o must not be used with kelvin or K). Since this temperature is also equal to 0.01 C, the temperature in kelvin is always equal to 273.15 plus the temperature in degrees Celsius. The unit was named after the British mathematician and physicist William Thomson (1824-1907), later known as Lord Kelvin.

    kilogram (kg)
    The SI base unit of mass is equal to the mass of the international prototype of the kilogram: a piece of platinum-iridium alloy kept at the International Bureau of Weights and Measures, Svres, France. One kilogram equals exactly 1000 gram and is approximately the mass of a litre of water.

    kilogram meter (kgfm)
    A metric unit of torque equal to 9.806 65 newton meters (Nm).

    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 newton. This unit is also called the kilopond.

    kip
    An old unit of force equal to 1000 pound-force or unit of mass equal to 1000 pounds or a symbol for 1000 inch pounds, used as a unit of energy or torque.

    knot (kn)
    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. One knot equals about 1.1508 miles per hour, exactly 1.852 kilometre per hour.

    light second
    A unit of distance equal to the distance light travels in a vacuum in one second. The light second distance is exactly 299 792 458 metre (about 186 282.4 miles). Similarly, a light minute is 60 light seconds (about 17 987 547 kilometre or 11 176 944 miles) and a light day is 1440 light minutes (about 24.902 billion kilometre or 16.095 billion 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. Light travels at 299 792 458 meters per second, and there are 31 556 925.9747 seconds in a year, so one light year equals 9.460 528 405 x 1015 metre, or 9.460 528 405 x 1012 kilometre. One light year is approximately 5.880 trillion miles.

    litre (L or l)
    The metric unit of volume. The litre was originally defined to be the volume occupied by a kilogram of water and equal to one cubic decimetre. In the SI, one kilogram water occupies about 1.000 028 cubic decimetre. To counter this discrepancy, the SI states that the litre "may be employed as a special name for the cubic decimetre." and defined as exactly 1 cubic decimetre, 1000 cubic centimetre, or 0.001 cubic metre.

    lumen (lm)
    The SI derived unit for measuring the luminous flux of light being produced by a light source or received by a surface. The luminous intensity of a light source is measured in candela. One lumen represents the total flux of light emitted, equal to the intensity in candela multiplied by the solid angle in steradians (1/(4pi) 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 4pi lumen.

    lux (lx)
    The SI derived unit for measuring the illumination of a surface. One lux is defined as an illumination of one lumen per square metre or 0.0001 phot.

    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 C and a pressure of 1 atmosphere the speed of sound is about 331.6 m/sec, or 741.8 mi/hr).

    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 centimetre perpendicular to the field. This unit was named after the British physicist James Clerk Maxwell (1831-1879).

    metre or meter (m)
    The SI base unit of length. Originally, the metre was designed to be one ten-millionth of a quadrant, the distance between the Equator and the North Pole. For a long time, the metre was defined as 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 base units (with one exception) have been redefined in abstract terms so they can be reproduced to very high levels of accuracy in well-equipped laboratories. The SI definition of the metre is the length of the path travelled by light in vacuum during a time interval of 1/299 792 458 of a second. The speed of light in a vacuum, c, (299 792 458 metre per second) is one of the fundamental constants of nature.

    metre per second
    The SI derived unit of velocity.

    metre per second per second
    The SI derived unit of acceleration (m s-2).

    micron ()
    A metric unit of distance equal to one millionth of a meter. The micron is simply a shorter name for the micrometre and is not an approved SI unit.

    mil
    A unit of angle measure, used in the military for artillery settings. The mil is equal to 1/1600 right angle.

    mile (mi)
    A traditional unit of distance. The word comes from the Latin word for 1000, mille, because originally a mile was the distance a Roman legion could march in 1000 paces. The British defined the statute mile to be 8 furlongs, 80 chains, 320 rods, 1760 yards or 5280 feet. The statute mile is exactly 1609.344 metre. See also nautical mile.

    minute (min or ')
    A unit of time equal to 60 seconds or to 1/60 hour. The SI specifies min as the symbol for this time unit. The minute is not an SI unit, but is accepted for use.
    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. The SI recommends ' as the symbol for the arcminute. This unit is not an SI unit, but is accepted for use. The international standard ISO 31 requires that angles be stated in degrees and decimal fractions of the degree, without use of arcminutes and arcseconds.

    molar gas constant (R)
    A constant occurring in the universal gas equation, i.e. the equation of state of an ideal gas: pV = nRT . Also known as universal gas constant, usually denoted by symbol R. Here p is the pressure of gas, V the volume it ocupies, n the number of moles of gas, and T its temperature. It can be shown that R is an universal constant, equal for all gases. Real gases obey this equation only in an approximation of very diluted gases. The molar gas constant is equal to 8.314 472 J mol-1 K-1.

    molar mass (Mm)
    The SI derived unit for the mass of a mole of substance and is measured in kilogram per mole.

    molar volume (Vm)
    The SI derived unit for the volume of a mole of substance and is measured in cubic metre per mole. This mainly used for gases. The behaviour of gases under ordinary conditions 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. The molar volume is the volume one mole of gas occupies at standard temperature (273.16 kelvins, or 0 C) and standard pressure (1 atmosphere, or 101.325 kilopascal). The molar volume is equal to 22.414 litre. (Occasionally the term "molar volume" is used for the volume occupied by a mole of a substance which is not a gas and in such cases the molar volume will be different for each substance.)

    mole (mol)
    The SI base unit of the amount of a substance. The mole is defined as the amount of substance of a system which contains as many elementary entities as there are atoms in 0.012 kilogram of carbon 12. When the mole is used, the elementary entities must be specified and may be atoms, molecules, ions, electrons, other particles or specified groups of such particle. In this definition, it is understood that the carbon 12 atoms are unbound, at rest and in their ground state. The actual number of "elementary entities" in a mole is called Avogadro constant

    nautical mile (nmi, naut mi or NM)
    A unit of distance used primarily at sea and in aviation. The nautical mile is defined to be the average distance on the Earth's surface represented by one minute of latitude. The nautical mile to be exactly 1852 metre or 6076.115 49 feet, a distance known as the international nautical mile. The international nautical mile equals about 1.1508 statute miles.

    neper (Np)
    A unit expressing the ratio of two numbers as a natural logarithm; the ratio r corresponds to (1/2) ln r nepers. Quantities differ by 1 neper if one is e2 = 7.389056 times the other. One neper is equal to about 8.685 890 decibels, and in general n nepers equal 20n/(ln 10) decibels. The neper is accepted for use with SI units, and is used as a measure of field level, power level, sound pressure level etc.. The unit recognises the British mathematician John Napier (1550-1617), the inventor of the logarithm.

    newton (N)
    The SI derived 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 newton was named after Isaac Newton (1642-1727). 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 (Nm)
    An SI derived unit of torque (energy). Torque, the tendency of a force to cause a rotation, is the product of the force and the distance from the centre 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 report torque in newton meters rather than in joules. One newton meter is approximately 0.737 562 pound foot.

    normal litre (NL or Nl)
    A unit of mass for gases equal to the mass of 1 litre (0.035 3147 ft3) at a pressure of 1 atmosphere and at a standard temperature, often 0 C (32 F) or 20 C (68 F). Air flow is often stated in normal litre per minute (Nl/min).

    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 centimetre from a unit magnetic pole. A field of one oersted generates a magnetomotive force of 1 gilbert per centimetre of conductor. One oersted equals (1000/4π) A m-1. The oerstead was named after Hans Christian rsted (1777-1851).

    ohm (Ω )
    The SI unit of electric resistance. If a conductor connects two locations having different electric potentials, then a current flows through the conductor. The amount of the current depends on the potential difference and the resistance to the flow of current. This property of a circuit, the electric resistance, is measured in ohms. One ohm is the resistance that requires a potential difference of one volt per ampere of current. The unit was name after the German physicist Georg Simon Ohm (1787-1854).

    ounce (oz)
    The avoirdupois ounce a traditional unit of weight is 1/16 pound or about 28.3495 grams. The avoirdupois ounce also equals about 0.911 457 troy ounce.

    The troy ounce a traditional unit of weight , used in pharmacy and jewellery, is 1/12 troy pound, or about 31.1035 grams. Thus the troy ounce equals 1.09714 avoirdupois ounces. This unit is the traditional measure for gold and other precious metals; in particular, the prices of gold and silver quoted in financial markets are the prices per troy ounce. The troy ounce is divided into 20 pennyweight or into 8 troy drams. The troy ounce is sometimes abbreviated oz t or toz to distinguish it from the more common avoirdupois ounce.

    parsec (pc or psc)
    A non-metric unit of distance used in astronomy. One parsec is the distance at which a star would appear to shift its position by one arcsecond during 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 parallax, in arcseconds. If the parallax is 0.01 arcsecond, the distance is 100 parsecs. One parsec divided by one astronomical unit (the length of the semimajor axis of the Earth's elliptical 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 petametre (30.856 78 x 1012 kilometre).

    pascal (Pa)
    The SI derived unit of pressure. The pascal is equal to one newton per square metre or one "kilogram per metre per second per second.

    pascal second (η) (Pa s)
    The SI derived unit of dynamic viscosity. The pascal second or kg m-1s-1is equivalent to 10 poise

    phon
    A logarithmic measure 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.

    phot (ph)
    The CGS unit of illuminance or illumination, equal to one lumen per square centimetre or 10 000 lux.

    pint (pt)
    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: the U. S. liquid pint, equal to 28.875 cubic inches, 16 fluid ounces, or approximately 0.473 176 litre; the U. S. dry pint, equal to 33.600 cubic inches or 0.550 611 litre; and the British imperial pint, equal to 20 British fluid ounces, 34.678 cubic inches, 1/8 imperial gallon or 0.568 261 litre.

    planck constant (h)
    A unit of "action" (energy expended over time). The constant of proportionality, represented by the symbol h, that relates the energy E of a photon with the frequency η of the associated wave through the relation E = , where planck constant h = 6.626 0693 x 10-34 joule second (Js). The unit honours the German physicist Max Planck (1858-1947).

    planck length (lp)
    A unit of distance representing the scale at which gravity, and perhaps space itself, becomes quantized (discrete) rather than continuous. This is the shortest distance that is meaningful in our understanding of the laws of physics. The planck length is defined to be the square root of Gh/c3, where G is the universal gravitational constant, h is Planck's constant, and c is the speed of light. This makes the planck length about 1.616 24 x 10-35 metre.

    planck time (tp)
    A unit of time equal to the time required for a photon moving at the speed of light to travel the distance of one planck length. This is the shortest time that is meaningful in our understanding of the laws of physics, representing the scale at which time itself may become quantized (discrete) rather than continuous. The planck time is about 5.391 21 x 10-44 second.

    poise (P, Ps, or Po)
    A CGS unit of dynamic viscosity equal to one dyne-second per square centimetre; the viscosity of a fluid in which a force of one dyne per square centimetre maintains a velocity of 1 centimetre per second. The unit poise is equivalent to 0.1 Pa s. in SI units.

    poiseuille (Pl)
    An MKS unit of dynamic viscosity equal to 1 pascal second or 10 poise. The poiseuille has been proposed, but not accepted, as an SI derived unit.

    pond (p)
    A metric unit of force. One pond is the gravitational force on a mass of one gram and it is equal to 980.665 dyne or 9.806 65 x 10-3newton. The kilopond was used more often than the pond. The name of the unit is from the Latin pondus, weight.

    pound (lb, lbm)
    A unit of mass or weight. The avoirdupois pound is divided into 16 ounces. By international agreement, one avoirdupois pound is equal to exactly 0.453 592 37 kilogram and this is exactly 175/144 = 1.215 28 troy pounds.

    pound foot (lbf ft or lb ft)
    A 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 centre 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 1.355 818 newton meter (Nm) in SI units.

    pound force (lbf)
    A unit of force. Traditional measuring systems did not distinguish between force and mass units. Pound force is the gravitational force experienced at the Earth's surface by a mass of one pound. Following Newton's law F = ma; one pound of mass is 0.453 592 kilogram multiplied by the acceleration due to gravity 9.806 65 meters per square second equals 4.448 221 615 newtons. The symbol lbf should be used for the pound force to distinguish it from the pound of mass.

    pound per square inch (lbf/in2 or psi)
    A unit of pressure. 1 psi equals 6.894 75 kilopascal (kPa)

    poundal (pdl or pl)
    An old English unit of force used in engineering. When traditional measuring systems did not distinguish between force and mass units, the poundal was defined to provide a unit clearly measuring force rather than mass. One poundal is the force that accelerates a mass of one pound at the rate of one foot per second.

    rad (rd)
    A metric unit for radiation dose. One rad is equal to a dose of 0.01 joule per kilogram (J/kg). The SI unit of radiation dose is the gray (Gy); one rad equals 0.01 gray or 10 milligrays.

    radian (rad)
    The SI derived unit of plane angle. One radian is the angle at the centre of a circle that cuts off an arc of length equal to the radius. Since the circumference equals 2 pi times the radius, one radian equals 1/(2 pi) of the circle, or approximately 57.295 779. The radian is a dimensionless unit.

    radian per second (rad/s)
    The SI derived unit of angular velocity. One radian per second is equal to a rotation frequency of about 0.159155 s-1 (9.5493 rpm).

    degree Rankine (R)
    A unit of absolute temperature. 1 Rankine represents the same temperature difference as 1 Fahrenheit, but the zero point of the scale is set at absolute zero. The unit was named after the British physicist and engineer William Rankine (1820-1872).

    rem (rem)
    A unit used for measuring the effective (or "equivalent") dose of radiation received by a living organism. One rem is equal to 0.01 sievert (Sv), which equals the actual dose received in rads, 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 ionising radiation in humans.

    roentgen or rntgen (R)
    A non-metric unit used to measure the ionising ability of radiation. The biological effects of radiation are caused mainly by excessive ionisation within living cells, so it is important to measure this ionising ability of radiation. One roentgen equals a charge release rate of 258 microcoulombs per kilogram of air. The unit is named after the German physicist Wilhelm Konrad Rntgen (1845-1923).

    rydberg constant (R)
    Named after physicist Janne Rydberg, and is a physical constant discovered when measuring the spectrum of hydrogen, and building upon results from Anders Jonas ngstrm and Johann Balmer. Each chemical element has it's own Rydberg constant, but most commonly referred to is the "infinity" constant (10 973 731.568 525 m-1).

    second (s)
    The SI base unit of time. The SI definition of the second is the duration of 9 192 631 770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the cesium-133 atom. The second was originally defined as 1/86 400 mean solar day until astronomers discovered that the mean solar day is actually not constant.

    second (")
    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. The international standard ISO 31 recommends that angles be stated in degrees and decimal fractions of the degree, without use of arcminutes and arcseconds.

    siemens (S)
    the SI derived unit of electric conductance. 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. The unit was named after Werner von Siemens (1816-1892).

    sievert (Sv)
    The SI derived unit used for measuring the dose equivalent 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. The sievert has the same joules per kilogram in SI base units as the gray. The unit was name after the Swedish physicist Rolf Sievert (1898-1966).

    square metre (m2)
    The SI derived unit of area.

    steradian (sr)
    The SI derived unit of solid angle. The steradian is defined as the solid angle which, having its vertex at the centre of the sphere, cuts off an area equal to the square of its radius. So, 1 steradian has a projected area of 1 square meter at a distance of 1 meter, etc. A sphere contains 4 pi steradians.

    stilb (sb)
    A CGS unit of luminance, equal to one candela per square centimetre.

    stoke or stokes (St)
    A CGS unit of kinematic viscosity. The stokes is defined to be 1 cm2 s-1, equivalent to 10-4 m2 s-1 Kinematic viscosity is defined to be dynamic viscosity (see poise) divided by the density of the liquid.

    tesla (T)
    The SI derived unit of flux density (or field intensity) for magnetic fields (also called the magnetic induction). 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. The unit was name after Nikola Tesla (1856-1943).

    therm (thm)
    A 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 therm equals 105.4804 megajoule. The European Union's definition, made in 1979 using the more current IT Btu, is 105.5060 megajoule.

    thou
    A British name for what Americans call a mil: a unit of distance equal to 0.001 inch (25.4 micrometers).

    ton (tn or T or t)
    A 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.

    tonne (t)
    A metric unit of mass equal to 1000 kilogram. Also called the metric ton.

    torr (Torr)
    A non-metric unit of pressure equal to exactly 1/760 atmosphere 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 as 1 mmHg.

    volt (V)
    The SI derived unit of electric potential. Electric potential is defined as the amount of potential energy present per unit of charge. The unit of electric potential is the volt, representing a potential of one joule per coulomb of charge.

    watt (W)
    The SI derived unit of power. Power is the rate at which work is done, or the rate at which energy is expended. One watt is equal to one joule per second. This unit is used both in mechanics and in electricity. The unit was named after James Watt (1736-1819), the British engineer.

    watt hour (Wh)
    A 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 kilojoule (kJ) of energy.

    weber (Wb)
    The SI derived unit of magnetic flux. The magnetic flux in webers is equal to the potential difference, in volts, that would be created by collapsing the field uniformly to zero in one second (V s).

    week (wk)
    A traditional unit of time equal to seven days.

    X unit (Xu)
    A unit of distance formerly used for measuring the wavelength of x-rays and gamma rays. The X unit is approximately 1.0021 x 10-13 meter. The wavelength of these powerful forms of radiation is now measured in picometre (pm) or femtometre (fm).

    yard (yd)
    A unit of distance equal to 3 feet or 36 inches. Today one yard is officially equal to exactly 0.9144 metre.

    year (a or y or yr)
    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 summer solstice in the Southern Hemisphere and the winter solstice in the Northern Hemisphere. Astronomers call this unit the tropical year. There are 365.242 199 days in a tropical year, or 31 556 925.9747 seconds.