Definitions of SI base units
The values of base units are set by an international organisation.
Metre
The length of the path travelled by light in vacuum during a time interval of 1/299792458 of a second.
Kilogram
The mass of the international prototype of the kilogram.
Second
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 caesium 133 atom.
Ampere
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 a vacuum, would produce between these conductors a force equal to 2 x 10-7 newton per metre of length.
Kelvin
1/273.16 of the thermodynamic temperature of the triple point of water.
Mole
The amount of substance of a system which contains as many elementary entities as there are atoms in 0.012 kilogram of carbon 12. The elementary entities may be atoms, molecules, ions, electrons, other particles, or specified groups of such particles.
Candela
Luminous intensity (intensity of a light source as observed by the human eye).
The luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency 540 x 10 to the power 12 hertz and that has a radiant intensity in that direction of 1/683 watt per steradian.
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