| 1<sup>st</sup> (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. |
| carat = 200 mg adopted. |
| 5<sup>th</sup> (1913) |
International Temperature Scale proposed. |
| 6<sup>th</sup> (1921) |
Metre Convention revised. |
| 7<sup>th</sup> (1927) |
Consultative Committee for Electricity (CCE) created. |
| 8<sup>th</sup> (1933) |
Need for absolute electrical unit identified. |
| 9<sup>th</sup> (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 http://www.bipm.org/jsp/en/ViewCGPMResolution.jsp?CGPM=9&RES=7. The universal return to the Long Scale numbering system was proposed but not adopted. |
| 10<sup>th</sup> (1954) |
kelvin, standard atmosphere defined. International System of Units (metre, kilogram, second, ampere, kelvin, candela) began. |
| 11<sup>th</sup> (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. |
| 12<sup>th</sup> (1964) |
original definition of litre = 1 dm³ restored. atto- and femto- prefixes. |
| 13<sup>th</sup> (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. |
| 14<sup>th</sup> (1971) |
new SI base unit mole defined. pascal, siemens approved. |
| 15<sup>th</sup> (1975) |
peta- and exa- prefixes. gray and becquerel radiological units. |
| 16<sup>th</sup> (1979) |
candela, sievert defined. Both l and L provisionally allowed as symbols for litre. |
| 17<sup>th</sup> (1983) |
metre redefined in terms of the speed of light, but keeps same length. |
| 18<sup>th</sup> (1987) |
conventional values adopted for Josephson constant, KJ, and von Klitzing constant, RK, preparing the way for alternative definitions of the ampere and kilogram. |
| 19<sup>th</sup> (1991) |
new prefixes yocto-, zepto-, zetta- and yotta-. |
| 20<sup>th</sup> (1995) |
SI supplementary units (radian and steradian) become derived units. |
| 21<sup>st</sup> (1999) |
new SI derived unit, the katal = mole per second, for the expression of catalytic activity. |
| 22<sup>nd</sup> (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". http://www.bipm.org/jsp/en/ViewCGPMResolution.jsp?CGPM=22&RES=10. |
| 23<sup>rd</sup> (2007) |
clarification about the kelvin and thoughts about possible revision of certain base units |
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