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International System of Units (view source)
Revision as of 17:05, June 29, 2007
, 17:05, June 29, 2007→Principles: Removed incorrect references to construction of derived units by addition and substraction of base units. You can't add meters to volts! .
==Principles==
==Principles==
The value of a physical quantity is considered to a number multiplied by a unit. Units may be '''base units''' or '''derived units'''. Derived units are constructed by adding, subtracting, multiplying or dividing base units or their multiples and submultiples. Thus a unit of speed, the meter per second, can be constructed by dividing the base unit of length, the meter, by the base unit for time, the second. The symbol for a derived unit is constructed from the symbols for its constituent base units. Multiplication can be denoted by a space or a the half-high dot ([[ANSI]] symbol 0183). Division is denoted by the '''solidus''' (oblique stroke) or by a negative exponent. For example, 25 meters per second could be written in symbols as 25 m/s or 25 m·s<sup>-1</sup>. New derived units can always be constructed as new situations arise. There is no limit to the number of additions, subtractions, multiplications and divisions that can be used, so there is no limit to the number of possible derived units. 22 of the derived units have their own names.
The value of a physical quantity is considered to a number multiplied by a unit. Units may be '''base units''' or '''derived units'''. Derived units are constructed by multiplying, dividing or raising to powers the base units or their multiples and submultiples. Thus a unit of speed, the meter per second, can be constructed by dividing the base unit of length, the meter, by the base unit for time, the second. The symbol for a derived unit is constructed from the symbols for its constituent base units. Multiplication should be denoted by either a space or the half-high dot ([[ANSI]] symbol 0183). Division is denoted by the '''solidus''' (oblique stroke) or by a negative exponent. For example, 25 meters per second could be written in symbols as 25 m/s or 25 m·s<sup>-1</sup>. New derived units can always be constructed as new situations arise. There is no limit to the number of multiplications, divisions and raising of powers that can be used, so there is no limit to the number of possible derived units. 22 of the derived units have their own names.
== The Base Units ==
== The Base Units ==
SI depends on seven ''base units'' of measurement from which all other units derive. They are:
SI depends on seven ''base units'' of measurement from which all other units derive. They are: