International System of Units
Le Systeme International d’Unites (SI units)
In 1960 the Conference Generale des Poids et Mesures (CGPM), which is the international authority on the metric system, accepted a universal, practical system of units and gave it the name Le Systeme International d’Unites with the abbreviation SI. Since then, this most modern and simplest form of the metric system was introduced throughout the world and by 1970’s more than 20 countries, including established metric countries, passed legislation adopting the SI system as their only legal system with numerous countries following their example.
THE SEVEN SI BASE UNITS |
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Quantity |
Name |
Sym |
Definition (CGPM) |
Length |
meter |
m |
The meter is the length equal to 1 650 763.73 wavelengths in vacuum of the radiation corresponding to the transition between the levels 2 p10 and 5 d5, of the krypton-86 atom. [11^{th} CGPM (1960), Resolution 6] |
Mass |
kilogram |
kg |
The kilogram is the mass of the international prototype of the kilogram recognized by the CGPM and in the custody of the Bureau International des Poids et Mesures, Sevres, France. [1^{st} CGPM (1880)] |
Time |
second |
s |
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 caesium-133 atom. [13^{th} CGPM (1967), Resolution 1] |
Electric current |
ampere |
A |
The ampere is that constant current which, if maintained in two straight parallel conductors of current infinite length, of negligible circular cross section, and placed one meter apart in vacuum would produce, between these conductors a force equal to 2 x 10^{-7} newton per meter of length. [CGPM (1946, Resolution 2, approved by the 9^{th} CGPM (1948)] |
Thermo-dynamic temperature |
kelvin |
K |
The kelvin, unit of thermodynamic temperature, is the fraction 1/273.16 of the thermodynamic temperature of the triple point of water. [13^{th} CGPM (1967), Resolution 4] |
Amount of substance |
mole |
mol |
The mole is the amount of substance of a system which contains as many elementary entities as there are atoms is 0.012 kg of carbon 12. [14^{th} CGPM (1971), Resolution 3] |
Luminous intensity |
candela |
cd |
The candela is the luminous intensity, in the perpendicular direction of a surface of 1/600 000 square meter of a blackbody at the temperature of freezing platinum under a pressure of 101 325 newtons per square meter. [13^{th} CGPM (1967), Resolution 5] |
Further readings:
http://physics.nist.gov/cuu/Units/meter.html