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International System of Units

Definitions of SI Base Units

The meter is the length of path travelled by light in vacuum during a time interval of 1/299 792 458 of a second (17th CGMP, 1983).
The kilogram is the unit of mass: it is equal to the mass of the international prototype of the kilogram (3rd CGMP, 1901)
The second is 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 cesium-133 atom (13th CGMP, 1967).
The ampere is that constant current which, if maintained in two straight parallel coductors of infinite length, of negligible circular cross-section, and placed 1 meter apart in vacuum, would produce between these conductors a force equal to 2 x 10-7 newton per meter of length (9th CGPM, 1948)
The kelvin, unit of thermodynamic temperature, is the fraction 1/273,16 of the thermodynamic temperature of the triple point of water (13th CGMP)
The mole is the amount of substances of a system which contains as many elementary entities as there are atoms in 0,012 kilogram of carbon-12. When the mole is used, the elementary entities must be specified and may be atoms, molecules, ions, electrons, other particles, or specific groups of such particles (14th CGPM,1971).
The candela is the luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency 540 x 1012 hertz and that has a radiant intensity in that direction of (1/683) watt per steradian (16th CGPM, 1979).

Names and Symbols for the SI Base Units

Physical quantity Name of SI unit Symbol for SI unit
length meter m
mass kilogram kg
time second s
electric current ampere A
thermodynamic temperature kelvin K
amount of substance mole mol
luminous intensity candela cd

SI Derived Units with Special Names and Symbols

Name of Symbol for Expression in
Physical Quantity SI unit SI Unit terms of SI base units
frequency hertz Hz s-1
force newton N m kg s-2
pressure, stress pascal Pa N m-2 = m-1 kg s-2
energy, work, heat joule J N m = m2 kg s-2
power, radiant flux watt W J s-1 = m2 kg s-3
electric charge coulomb C A s
electric potential, volt V J C-1 = m2 kg s-3 A-1
(electromotive force)
electric resistance ohm W V A-1 = m2 kg s-3 A-2
electric conductance siemens S W-1 = m-2 kg-1 s3 A-2
electric capacity farad F C V-1 = m-2 kg-1 s4 A2
magnetic flux density tesla T V s m-2 = kg s-2 A-1
magnetic flux weber Wb V s = m2 kg s-2 A-1
inductance henry H V A-1 s = m2 kg s-2 A-2
Celsius temperature degree Celsius °C K
luminous flux lumen lm cd sr
illuminance lux lx cd sr m-2
activity (radioactice) becquerel Bq s-1
absorbed dose (of radiation) gray Gy J kg-1 = m2 s-2
dose equivalent sievert Sv J kg-1 = m2 s-2
(dose of equivalent index)
plane angle radian rad 1 = m m-1
solid angle steradian sr 1 = m2 m-2

SI Prefixes

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

© 10.02.2003 by Florian Häußermann. No rights reserved.