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 AndréMarie Ampère (17751836). The
ampere is defined as the constant current which, if maintained in two straight
parallel conductors of infinite length, of negligible circular crosssection,
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 (A·h 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 (Å) A
metric unit of length, equal to 0.1 nanometre or 10^{10} metre.
Angstrom unit named after the Swedish physicist Anders Jon Ångström (18141874)
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
10^{11} 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 10^{5} 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 wholenumber 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 carbon12 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 10^{23} 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 (17761856). 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 10^{5} 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,
AntoineHenri Becquerel (18521908), 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 (18471922). 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 (a_{0}) 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 (18851962).

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.

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:
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, CharlesAugustin de Coulomb
(17361806).

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

cubic metre (m^{3}) 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 10^{10} 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 (17011744). 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.

electronvoltkilogram relationship ((1 eV)/c^{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 electronvolt is therefore equivalent to a mass of about 1.782 661 81
x 10^{36} kg.

electron mass (m_{e}) 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 10^{5} 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 (16861736).
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. 1°F 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 (17911867).

faraday (Fd) A unit of electric charge. The British
electrochemist and physicist Michael Faraday (17911867) 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 g_{n} 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 105 oersted (10 µOe). 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 crosssection perpendicular to the field. In SI units,
one gauss equals 104 tesla. The
unit was named after Karl Friedrich Gauss (17771855).

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=Gm_{1}m_{2}/r^{2}. G = 6.6742(10)
x 10^{11} m^{3} kg^{1} s^{2} or N
m^{2} 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 (19051965). 
hartree (Eh) A unit of energy used in nuclear
physics, equal to about 4.3598 x 1018 joule or
27.212 electron
volts. The unit was named after the British physicist and mathematician
Douglas R. Hartree (18971958).

hectare (ha) A
metric unit of land area, equal to 100 are. One
hectare is a square hectometre (10 000 m^{2}) 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 (17971878).

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 (18571894). 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 (17361819) who determined that a horse is typically
capable of a power rate of 550 footpounds
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
(19051950), 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 (18181889). 
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=TT_{0} where T_{0}=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 (18241907), 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 platinumiridium alloy kept at the International Bureau of
Weights and Measures, Sévres, France. One kilogram equals exactly 1000 gram and
is approximately the mass of a litre of
water.

kilogram meter (kgf·m) A
metric unit of torque equal to 9.806 65 newton meters
(N·m).

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 poundforce 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 10^{15} metre, or 9.460 528 405 x 10^{12} 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/(4·pi) of a sphere) into
which the light is emitted. Thus the total flux of a onecandela light, if the
light is emitted uniformly in all directions, is 4·pi 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 (18311879).

metre or meter (m) The
SI base unit of length. Originally, the metre was designed to be one
tenmillionth 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 wellequipped 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 (µ) 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 (M_{m}) The SI
derived unit for the mass of a mole of
substance and is measured in kilogram per mole.

molar volume (V_{m}) 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 (15501617), 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 (16421727). 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) 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 ft^{3}) 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 Ørsted (17771851).

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
(17871854).

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
nonmetric 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^{1}s^{1}is 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 = hη, where
planck constant h = 6.626 0693 x 10^{34} joule second (J·s). The
unit honours the German physicist Max Planck (18581947).

planck length (l_{p}) 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/c^{3}, 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 (t_{p}) 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 dynesecond 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^{3}newton.
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
(N·m) 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/in^{2} 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 (18201872). 
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 röntgen (R) A nonmetric 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 Röntgen
(18451923). 
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 Ångström 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 cesium133 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 (18161892).

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 (18981966).

square metre (m^{2}) 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 cm^{2}
s^{1}, equivalent to 10^{4} m^{2} 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 (18561943).

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
nonmetric 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 (17361819), the British engineer.

watt hour (W·h) 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 xrays 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. 