General Conference On Weights and Measures - CGPM Meetings

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 by lot 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 . 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 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 conductivity 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 principal subject to certain technical criteria having been met.

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