Table of S.I. Standards

Definition of the S.I. standards

Unit	Definition	Last Revision
metre	The length of the path travelled by light in vacuum during a time interval of 1/(299 792 458) of a second.	1983
kilogram	A unit of mass; it is equal to the mass of the international prototype of the kilogram.	1901
second	The duration of 9 192 631 770 periods of radiation corresponding to the transistion between the two hyperline levels of the ground state of the caesium-133 atom.	1967
ampere	The constant current which, if maintained in two straight parallel conductors of infinite length, of negligible cross-section, and placed one metre apart in vacuum, would produce between the conductors a force equal to 2 x 10^(-7) newton/metre.	1948
kelvin	A unit of thermodynamic temperature. The kelvin is the fraction 1/273.16 of the thermodynamic temperature of the triple point of water.	1967
candela	The candela is the luminosity intensity, in a given direction, of a source that emits monochromatic radiation of the frequency 540 x 10^12 hertz and that has a radiant intensity in that direction of 1/683 watt per steradian.	1979
mole	The mole is the amount of substance in a system which contains as many elementary entities as there are carbon atoms in 0.012 kilogram of carbon-12. The elementary entities must be specified and may be atoms, molecules, ions, electrons, other particles, or specified groups of such particles	1971

Notes

• Some units are defined in terms of other units. For example, the second in the definition of the metre.

• The definition of the kilogram is by far the oldest and is the only unit that "suffers" from having an international prototype. That means that somewhere in the world there is a block of mass that the international scientific community had agreed weighs exactly one kilogram. Every other weight is measured relative to this prototype. Consequently, if somebody dropped the prototype and a little bit fell off, everything else would weigh a little bit more (in kilograms), since the definition of a kilogram would be a smaller mass.

• The triple point of water that is mentioned in the definiton of the kelvin, is the temperature at which ice, liquid water and steam can all exist simultaneously. You may recognise this by its more familiar temperature of zero degrees Celsius.

• The steridian that is mentioned in the definition of the candela is a three-dimensional analogue of an angle. In effect, if we take a sphere of unit radius, then a segment of one steridian covers one quarter of the surface area of the sphere.