 |
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 |