General Conference on Weights and Measures

For a topical guide to this subject, see Outline of the metric system.

The General Conference on Weights and Measures (French: Conférence générale des poids et mesures - CGPM) is the senior of the three Inter-governmental organizations established in 1875 under the terms of the Metre Convention (French: Convention du Mètre) to represent the interests of member states. The treaty, which also set up two further bodies, the International Committee for Weights and Measures (French: Comité international des poids et mesures- CIPM) and the International Bureau of Weights and Measures (French: Bureau international des poids et mesures - BIPM), was drawn up to coordinate international metrology and to coordinate the development of the metric system.

The conference meets in Sèvres (south-west of Paris) every four to six years. Initially it was only concerned with the kilogram and the metre, but in 1921 the scope of the treaty was extended to accommodate all physical measurements and hence all aspects of the metric system. In 1960 the 11th CGPM approved the Système International d'Unités, usually known as "SI".

Establishment

On 20 May 1875 an international treaty known as the Convention du Mètre (Metre Convention) was signed by 17 states. This treaty established the following organisations to conduct international activities relating to a uniform system for measurements:

• Conférence générale des poids et mesures (CGPM), an intergovernmental conference of official delegates of member nations and the supreme authority for all actions;
• Comité international des poids et mesures (CIPM), consisting of selected scientists and metrologists, which prepares and executes the decisions of the CGPM and is responsible for the supervision of the International Bureau of Weights and Measures;
• Bureau international des poids et mesures (BIPM), a permanent laboratory and world centre of scientific metrology, the activities of which include the establishment of the basic standards and scales of the principal physical quantities and maintenance of the international prototype standards.

The CGPM acts on behalf of the governments of its members. In so doing, it appoints members to the CIPM, receives reports from the CIPM which it passes on to the governments and national laboratories on member states, examines and where appropriate approves proposals from the CIPM in respect of changes to the International System of Units (SI), approves the budget for the BIPM (over €10 million in 2012) and it decides all major issues concerning the organization and development of the BIPM.

Membership criteria

The CGPM recognises two classes of membership - full membership for those states that wish to participate in the activities of the BIPM and associate membership for those countries or economies that only wish to participate in the MRA program. Associate members have observer status at the CGPM. Since all formal liaison between the convention organisations and national governments is handled by the member state's ambassador to France, it is implicit that member states must have diplomatic relations with France, though during both world wars, nations that were at war with France retained their membership of the CGPM. The opening session of each CGPM is chaired by the French foreign minister and subsequent sessions by the Président de l'Académie des Sciences de Paris.

Of the twenty countries that attended the Conference of the Metre in 1875, representatives of seventeen signed the convention on 20 May 1875. In April 1884 HJ Chaney, Warden of Standards in London unofficially contacted the BIPM inquiring whether the BIPM would calibrate some metre standards that had been manufactured in the United Kingdom. Broch, director of the BIPM replied that he was not authorised to perform any such calibrations for non-member states. On 17 September 1884, the British Government signed the convention on behalf of the United Kingdom. This number grew to 21 in 1900, 32 in 1950, and 49 in 2001. As of 7 August 2013, there are 55 Member States and 38 Associate States and Economies of the General Conference (with year of partnership in parentheses):

Member States

Argentina (1877)
Australia (1947)
Austria (1875)
Belgium (1875)
Brazil (1921)
Bulgaria (1911)
Canada (1907)
Chile (1908)
China (1977)
Colombia (2012)
Croatia (2008)
Czech Republic (1922)
Denmark (1875)
Dominican Republic (1954)
Egypt (1962)
Finland (1923)
France (1875)
Germany (1875)
Greece (2001)
Hungary (1925)
India (1957)
Indonesia (1960)
Iran (1975)
Ireland (1925)
Israel (1985)
Italy (1875)
Japan (1885)
Kazakhstan (2008)
Kenya (2010)
Malaysia (2001)
Mexico (1890)
Netherlands (1929)
New Zealand (1991)
Norway (1875)
Pakistan (1973)
Poland (1925)
Portugal (1876)
Romania (1884)
Russia (1875)
Saudi Arabia (2011)
Serbia (2001)
Singapore (1994)
Slovakia (1922)
South Africa (1964)
South Korea (1959)
Spain (1875)
Sweden (1875)
Switzerland (1875)
Thailand (1912)
Tunisia (2012)
Turkey (1875)
United Kingdom (1884)
United States (1878)
Uruguay (1908)
Venezuela (1879)

Notes

Associates

At its 21st meeting (October 1999), the CGPM created the category of "associate" for those states not yet members of the BIPM and for economic unions.

Albania (2007)
Bangladesh (2010)
Belarus (2003)
Bolivia (2008)
Bosnia and Herzegovina (2011)
Botswana (2012)
Caribbean Community (2005)
Chinese Taipei (2002)
Costa Rica (2004)
Cuba (2000)
Ecuador (2000)
Estonia (2005)
Georgia (2008)
Ghana (2009)
Hong Kong (2000)
Jamaica (2003)
Latvia (2001)
Lithuania (2001)
Macedonia (2006)
Malta (2001)
Mauritius (2010)
Mongolia (2013)
Montenegro (2011)
Namibia (2012)
Oman (2012)
Panama (2003)
Paraguay (2009)
Peru (2009)
Philippines (2002)
Moldova (2007)
Seychelles (2010)
Slovenia (2003)
Sri Lanka (2007)
Syria (2012)
Ukraine (2002)
Vietnam (2003)
Zambia (2010)
Zimbabwe (2010)

CGPM meetings

1st (1889)	The International Prototype Kilogram (IPK), a cylinder made of platinum-iridium and the International Prototype Metre, an X-cross-section bar also made from platinum-iridium were selected from batches manufactured by the British firm Johnson Matthey. Working copies of both artifacts were also selected by lot and other copies distributed to member nations, again by lot. The prototypes and working copies were deposited at the International Bureau of Weights and Measures (Bureau international des poids et mesures), Sèvres, France.
2nd (1897)	No resolutions were passed by the 2nd CGPM.
3rd (1901)	The litre was 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)	The carat was defined as 200 mg.
5th (1913)	The International Temperature Scale was proposed.
6th (1921)	The Metre Convention revised.
7th (1927)	The Consultative Committee for Electricity (CCE) created.
8th (1933)	The need for absolute electrical unit identified.
9th (1948)	The ampere, bar, coulomb, farad, henry, joule, newton, ohm, volt, watt, weber were defined. The degree Celsius was selected from three names in use as the name of the unit of temperature. The symbol l (lowercase L) was 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 [2]. The universal return to the Long Scale numbering system was proposed but not adopted.
10th (1954)	The kelvin, standard atmosphere defined. Work on the International System of Units (metre, kilogram, second, ampere, kelvin, candela) began.
11th (1960)	The metre was redefined in terms of wavelengths of light. The Units hertz, lumen, lux, tesla were adopted. The new MKSA-based metric system given the official symbol SI for Système International d'Unités and launched as the "modernized metric system". The prefixes pico-, nano-, micro-, mega-, giga- and tera- were confirmed.
12th (1964)	The original definition of litre = 1 dm3 restored. The prefixes atto- and femto- were adopted.
13th (1967)	The second was 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. The Degree Kelvin renamed kelvin and the candela redefined.
14th (1971)	A new SI base unit, the mole defined. The names pascal and siemens as units of pressure and electrical conductance were approved.
15th (1975)	The prefixes peta- and exa- were adopted. The units gray and becquerel were adopted as radiological units within SI.
16th (1979)	The candela and sievert were defined. Both l and L provisionally allowed as symbols for litre.
17th (1983)	The metre was redefined in terms of the speed of light.
18th (1987)	Conventional values were 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- were adopted.
20th (1995)	The SI supplementary units (radian and steradian) become derived units.
21st (1999)	A new SI derived unit, the katal = mole per second, was adopted as the SI unit 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".
23rd (2007)	The definition of the kelvin was clarified and thoughts about possible revision of certain base units discussed.
24th (2011)	Proposal to revise the definitions of the SI units, including redefining the kilogram in relation to the Planck constant were accepted in principle, subject to certain technical criteria having been met.
25th (2014)	Redefining the kilogram in relation to the Planck constant was discussed but not decided on. Progress towards realising the redefinition has been noted. However, it was concluded that the data did not yet appear to be sufficiently robust. Continued effort on improving the data has been encouraged, such that a resolution that would replace the current definition with the revised definition can be adopted at the 26th meeting.

See also

• International Bureau of Weights and Measures (BIPM)
• International Committee for Weights and Measures (CIPM)
• Institute for Reference Materials and Measurements (IRMM)
• National Institute of Standards and Technology (NIST)
• National Conference on Weights and Measures (NCWM) United States