Difference between revisions of "International System of Units"

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Revision as of 19:15, 21 March 2010

The International System of Units (commonly abbreviated to SI from its French name Système Internationale d'Unités) is the international system of units of measurement established under the Metre Convention. It is based on seven "base units", with a potentially limitless number of "derived units".

Base units

Unit name Unit symbol Physical quantity Notes
metre m length
kilogram kg mass
second s time
ampere A electric current
kelvin K thermodynamic temperature
candela cd luminous intensity
mole mol amount of substance

The six original base units were selected in 1956 by the 9th CGPM (the mole was added in 1972). They represent a conventional choice: the choice made in the International System of Units is not the only possible choice of base units, nor is there any physical significance that the ampere was chosen as the base for electromagnetic units rather than, e.g., the coulomb.

The inclusion of the candela as a base unit is for both historical and practical reasons. There is an obvious technical need to classify many everyday light sources on the basis of their effect on the human eye. This is acheived by multiplying the radient power (measured in watts per steradian) by a standard luminosity function that models the response of the human eye: the result is the candela, which is also the base for a series of other luminosity units.

Derived units

Multiples and submultiples

Multiples   Submultiples
Symbol Prefix     Prefix Symbol
da deca 101 10−1 deci d
h hecto 102 10−2 centi c
k kilo 103 10−3 milli m
M mega 106 10−6 micro µ
G giga 109 10−9 nano n
T tera 1012 10−12 pico p
P peta 1015 10−15 femto f
E exa 1018 10−18 atto a
Z zetta 1021 10−21 zepto z
Y yotta 1024 10−24 yocto y

Units used with the SI

Notes and references

Notes

References

External links

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