Difference between revisions of "International System of Units"
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| + | 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]]. | ||
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| + | 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 [[watt]]s 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== | ==Derived units== | ||
Revision as of 19:01, 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".
Contents
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
Units used with the SI
Notes and references
Notes
References
External links
 |
See also the corresponding article on Wikipedia. |
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