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The following definitions of the SI base units are taken from
NIST Special Publication 330 (SP 330), The International System of Units (SI). See the Bibliography for a description of SP 330 and other NIST publications on the SI, and online access.
Definitions of the SI base units
|
| Unit of length |
meter |
 The meter is the length of the path travelled by light in vacuum
during a time interval of 1/299 792 458 of a second. |
|
| Unit of mass |
kilogram |
The kilogram is the unit of mass; it is equal to the mass of the
international prototype of the kilogram. |
|
| Unit of time |
second |
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. |
|
Unit of
electric current |
ampere |
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. |
|
Unit of
thermodynamic
temperature |
kelvin |
The kelvin, unit of thermodynamic temperature, is the fraction
1/273.16 of the thermodynamic temperature of the triple point
of water. |
|
Unit of
amount of
substance |
mole |
1. The mole is the amount of substance of a system which contains
as many elementary entities as there are atoms in 0.012 kilogram
of carbon 12; its symbol is "mol."
2. 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 particles. |
|
Unit of
luminous
intensity |
candela |
The candela is 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. |
|
Go to SI Units Background or SI base units |
