General Conference on Weights and Measures

The General Conference on Weights and Measures is the English name of the Conférence générale des poids et mesures (CGPM, never GCWM). It is one of the three organizations established to maintain the International System of Units (SI) under the terms of the Convention du Mètre (Metre Convention) of 1875. It meets in Sèvres (in the southwestern suburbs of Paris) every four to six years. The CGPM represents 52 member states and 26 further associate members.^[1]

CGPM meetings

1st (1889)	kilogram defined as mass of the international prototype kilogram (IPK) made of platinum-iridium and kept at the International Bureau of Weights and Measures (Bureau international des poids et mesures), Sèvres, France. International prototype metre sanctioned.
2nd (1897)	No resolutions were passed by the 2nd CGPM.
3rd (1901)	litre redefined as volume of 1 kg of water. Clarified that kilograms are units of mass, "standard weight" defined, standard acceleration of gravity defined endorsing use of grams force and making them well-defined.
4th (1907)	carat = 200 mg adopted.
5th (1913)	International Temperature Scale proposed.
6th (1921)	Metre Convention revised.
7th (1927)	Consultative Committee for Electricity (CCE) created.
8th (1933)	Need for absolute electrical unit identified.
9th (1948)	ampere, bar, coulomb, farad, henry, joule, newton, ohm, volt, watt, weber defined. Chose degree Celsius from among the three names then in use. l (lowercase L) adopted as symbol for litre. Both the comma and dot on a line are accepted as decimal marker symbols. Symbols for the stere and second changed [1]. The universal return to the Long Scale numbering system was proposed but not adopted.
10th (1954)	kelvin, standard atmosphere defined. International System of Units (metre, kilogram, second, ampere, kelvin, candela) began.
11th (1960)	metre redefined in terms of wavelengths of light. Units: hertz, lumen, lux, tesla adopted. New metric system given the official symbol SI for Système International d'Unités, the "modernized metric system". Prefixes pico-, nano-, micro-, mega-, giga- and tera- confirmed.
12th (1964)	original definition of litre = 1 dm3 restored. atto- and femto- prefixes.
13th (1967)	second redefined as 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 at a temperature of 0 K. Degree Kelvin renamed kelvin. Candela redefined.
14th (1971)	new SI base unit mole defined. pascal, siemens approved.
15th (1975)	peta- and exa- prefixes. gray and becquerel radiological units.
16th (1979)	candela, sievert defined. Both l and L provisionally allowed as symbols for litre.
17th (1983)	metre redefined in terms of the speed of light, but keeps same length.
18th (1987)	conventional values adopted for Josephson constant, KJ, and von Klitzing constant, RK, preparing the way for alternative definitions of the ampere and kilogram.
19th (1991)	new prefixes yocto-, zepto-, zetta- and yotta-.
20th (1995)	SI supplementary units (radian and steradian) become derived units.
21st (1999)	new SI derived unit, the katal = mole per second, for the expression of catalytic activity.
22nd (2003)	A comma or a dot on a line are reaffirmed as decimal marker symbols, and not as grouping symbols in order to facilitate reading; "numbers may be divided in groups of three in order to facilitate reading; neither dots nor commas are ever inserted in the spaces between groups".[2]
23rd (2007)	clarification about the kelvin and thoughts about possible revision of certain base units

See also

• International Bureau of Weights and Measures (BIPM)
• International Committee for Weights and Measures
• Institute for Reference Materials and Measurements (IRMM)
• National Institute of Standards and Technology (NIST)

References

1. ^ CGPM Member States
2. ^ http://www.bipm.org/jsp/en/ViewCGPMResolution.jsp?CGPM=22&RES=10