Dictionary of Principal Units

What follows is meant to be a dictionary of units that arise in Physics, everyday life, and academic competitions. It is

This dictionary is thickly, insanely, and, perhaps annoyingly cross-referenced. Selecting an underlined unit will take you to the dictionary entry for that unit.

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- ABAMPERE
- A unit of current. 1 abampere = 10 amperes.

- ACRE
- A unit of area in the "English" system of units. Originally,
the acre was one of a family of units of measure used to represent
the amount of land that a man could plow by hand in a unit of
time, usually a day. Today, the acre equals 43 560 feet
^{2}or 4 840 yard^{2}.

- AMPERE (A)
- The SI unit of current. The 9th General Conference on Weights
and Measures (CGPM -1948) adopted the ampere for the unit of
electric current, with the following definition:

The ampere is that constant current which, if maintained in two straight parallel conductors of infinite length, of negligible circular cross section, and placed 1 meter apart in vacuum, would produce between these conductors a force equal to 2 x 10^{-7}newton per meter of length.

- AMPERE PER METER (A/m)
- The SI unit of magnetic field strength. One ampere/meter is
the magnetic field strength in the interior of an elongated,
uniformly-wound solenoid which is excited with a linear current
density in its winding of 1 ampere per
meter of axial distance.

- AMPERE
^{.}HOUR (Ah) - A unit of amount of electrical charge. An
ampere
^{.}hour is the quantity of electricity represented by a current of 1 ampere flowing for 1 hour.

- ANGSTROM (A with a circle on top)
- A unit of length. One angstrom equals 10
^{-10}meter.

- APOSTILB (asb)
- A unit of luminance. One lumen
per square meter leaves a
surface whose luminance is 1 apostilb in all directions within a
hemisphere.

- ARE
- An SI unit of area. One are equals 100 meter
^{2}(the area of a 10 meter x 10 meter square). The hectare (= 100 are = 10^{4}meter^{2}) is more commonly used.

- ASTRONOMICAL UNIT (au)
- An SI unit of length equal to the mean radius of the Earth's
orbit. One astronomical unit equals 1.495 x 10
^{11}meters.

- ATMOSPHERE, STANDARD (atm)
- A unit of pressure. One standard atmosphere equals 101 325
newtons per square
meter.

- ATOMIC MASS UNIT, UNIFIED (u)
- The atomic mass unit (unified) is 1/12th of the mass of an
atom of the carbon-12 (
^{12}C) nuclide.

- BAR (bar)
- A unit of pressure. One bar equals 100 000 (10
^{5}) newtons per square meter.

- BARN (b)
- A unit of nuclear cross section (area). One barn equals
10
^{-28}square meter.

- BARREL (bbl)
- A unit of volume. One barrel equals 9 702 cubic inches;
or 0.158 99 cubic meters.

- BAUD (Bd)
- A unit of signaling speed. One baud equals one element (
bit per second.

- BEL (B)
- A dimensionless unit for expressing the ratio of two values of
power, being the logarithm to the base 10 of the power ratio. (The
more commonly used unit, decibel (dB), is
10 times the logarithm to the base 10 of the power ratio. A bel is
10 decibel.)

- BIT (b)
- A unit of information, generally represented by a pulse. A bit
is a binary digit, i.e., a 1 or 0 in computer technology.

- BIT PER SECOND (b/s)
- A unit of signaling speed. A transference rate of 1 bit per
second. One bit/second = 1 baud.

- BRITISH THERMAL UNIT (Btu)
- A unit of heat. The heat required to warm 1 pound of pure
water through an interval of 1 degree Fahrenheit.

- CALORIE (International Table) (cal
_{It}) - A unit of heat. One International Table calorie equals 4.1868
joules. (The 9th Conference
Generale des Poids et Mesures adopted the joule
as the unit of heat.)

- CALORIE (Thermochemical Calorie) (cal)
- A unit of heat. One calorie equals 4.1840 joules.

- CANDELA (cd)
- The SI unit of luminous intensity. The unit based on flame or
incandescent filament standards before 1948 was replaced initially
by a unit based on a Planckian radiator (a black body) at the
temperature of freezing platinum. Because of difficulties with
this measurement and new experimental techniques, the 16th General
Conference on Weights and Measures (CGPM 1979) adopted the
following definition:

The candela is the luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency 540 x 10^{12}hertz and that has a radiant intensity in that direction of 1/683 watt per steradian.

This unit was formerly called the candle.

- CANDLE (c)
- An antiquated unit of luminous intensity. Use of the SI unit,
the candela, is preferred.

- CELSIUS (
^{o}C) - A unit of temperature. See DEGREE
CELSIUS

- CENTIMETER (cm)
- The CGS unit of length. One centimeter equals (1) 0.01
inches.

- CENTIPOISE (cP)
- A unit of dynamic viscosity. 1 centipoise = 10
^{-2}poise.

- CENTISTOKE (cSt)
- A unit of kinematic viscosity. 1 centistoke equals
10
^{-2}stokes.

- CIRCULAR MIL (cmil)
- The area of a circle whose diameter is 0.0001
(10
^{-4}) inch. One circular mil equals I.4 x10^{-6}square inches.

- COULOMB (C)
- The quantity of electric charge which passes any cross section
of a conductor in 1 second when the current is maintained constant
at 1 ampere. The coulomb is the SI unit of
electric charge. (See also Fundamental
Units)

- CUBIT
- A historically-famous unit of length. One cubit was originally
defined as the distance from a person's elbow to the end of their
outstretched fingertips. Various cubit-like lengths existed
throughout the ancient world. One cubit is approximately 20
inches.

- CUP (c)
- A unit of volume in the "English" system. One cup = 0.5
pint. The cup is
**not**recommended for scientific purposes.

- CURIE (Ci)
- The unit of activity in the field of radiation dosimetry. One
curie equals 3.7 x 10
^{10}disintigrations per second.

- CYCLE (c)
- An interval of space of time in which is completed 1 round of
events or phenomena.

- CYCLE PER SECOND (Hz, c/s)
- The number of cycles per second. (The name hertz
(Hz) is the accepted international term. The abbreviation Hz is
preferred to c/s.)

- DARCY (D)
- A unit of permeability of a porous medium. One darcy equals 1
cP( cm/
s)( cm/
atm) equals 0.986 923 square micrometers.
(A permeability of 1 darcy will allow the flow of 1 cubic
centimeter per second
of fluid of 1 centipoise viscosity
through an area of 1 square centimeter
under a pressure gradient of 1 atmosphere
per centimeter.)

- DAY (d)
- A unit of time, the exact definition of which is dependent
upon which system of time measurement is referred to, i.e.,
apparent solar time, mean solar time, universal time, apparent
sidereal time, ephemeris time, or atomic time. With exception of
atomic time, the time base is referenced to rotation of the earth.
For general purposes, a day is considered the period taken for 1
revolution of the earth about its axis.

- DECIBEL (db)
- A dimensionless unit for expressing the ratio of two values of
power, being 10 times the logarithm to the base 10 of the power
ratio. A 1 decibel equals 10 bel.)

- DEGREE CELSIUS (
^{o}C) - A unit of temperature. The Celsius temperature scale is derived from the thermodynamic or Kelvin scale of temperature. They are related by: Celsius temperature equals Kelvin temperature minus 273.15 (C = K - 273.15). Some commonly used Celsius temperatures are:
- DEGREE FAHRENHEIT (
^{o}F) - A unit of temperature. The Fahrenheit temperature scale is
related to the Celsiius temperature scale by: Fahrenheit
temperature equals 1.8 x Celsius temperature plus 32 (F = 1.8C +
32
^{o}). Some commonly used Fahrenheit temperatures are:- absolute zero is -459.69
^{o}F - water freezes at 32
^{o}F - "room temperature" is approximately
70
^{o}F - "normal" body temperature is approximately
98.6
^{o}F - water boils at 212
^{o}F (at 1 atm)

- absolute zero is -459.69
- DEGREE KELVIN (
^{o}) - An outdated unit of temperature. The preferred unit is the
kelvin.

- DEGREE RANKINE (
^{o}R) - A unit of temperature. The Rankine temperature scale is
related to the Fahrenheit temperature scale by: Rankine
temperature equals Fahrenheit temperature minus 459.69

- DYNE (dyn)
- A unit of force. One dyne equals the force necessary to give a
1 gram mass an acceleration
of 1 centimeter per second
per second. 1 dyne = 10
^{-5}newton. The dyne is the unit of force in the CGS system.

- ELECTRONVOLT (eV)
- A unit of energy. One electronvolt equals the energy acquired
by an electron when it passes through a potential difference of 1
volt in a vacuum. One
elctronvolt equals 1.602 x 10
^{-12}erg.

- ELECTROSTATIC UNIT (esu)
- A unit of quantity of electricity.

- ERG (erg)
- A unit of energy. One erg is the work done when a constant
force of 1 dyne is applied through a distance
of 1 centimeter. 1 erg = 10
^{-7}joule. Also, 1 erg = 7.376 x 10^{-8}foot^{.}pounds = 9.481 x 10^{-11}Btu. The erg is the unit of energy in the CGS system.

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last update October 7, 2000 by JL Stanbrough