Chronology - Key steps in the history of the International System of Units (SI)

17 April 1795 The law of 18 Germinal Year III (Republican calendar) established the Metric System in France.

22 June 1799 Two platinum standards representing the metre and the kilogram were deposited in the French National Archives.

1832 Carl Friedrich Gauss introduced a system of “absolute” units based on the millimetre, the milligram and the second.

1st Sept 1869 Emperor Napoleon III approved the creation of an international scientific commission to propagate the use of metric measurement to facilitate trade, the comparison of measurement between states and the creation of an international metre prototype.

16th Nov 1869 The French government invited counties to join the International Scientific Commission.

1870 The first meeting of the newly formed International Metre Commission.

1872 The decision was taken, by the International Metre Commission committee of preparatory research, to make porotype copies of the original standards deposited at the Archives de la Republique which took another 16 years.

1874 The British Association for the Advancement of Science introduced the CGS (centimetre, gram and second) System.

20 May 1875 The signing of the Metre Convention on 20th May, by 17 countries, established The General Conference on Weights and Measures (CGPM) to discuss and endorse proposed changes to the system of units and The International Committee for Weights and Measures (CIPM), to oversee the discussion and recommendations on the system of units and the establishment of The International Bureau of Weights and Measures (BIPM) to provide the administration of the entire system and to house the international prototypes standards. The system of units agreed was similar to the CGS but called MKS with base units of the metre, kilogram and second. The metre and kilogram were represented by physical artefacts and the second by the astronomical second.

1889 The first CGPM sanctioned the new international prototypes of the metre and the kilogram.

1901 Giovanni Giorgi proposed to the Associazione Elettrotecnica Italiana a new system enabling the combination of the fundamental units, the kilogram, the metre and the second, with a fourth unit of an electrical nature.

1921 Revision of the Metre Convention extending the activities of the BIPM to new fields of metrology.

1927 The Consultative Committee for Electricity (CCE, now the Consultative Committee for Electricity and Magnetism CEPM) was created by the CIPM. It was the first such committee.

1935 The International Electrotechnical Commission (IEC) adopted the Giorgi System known as the MKS System.

1939 The CCE recommended the adoption of the MKS System based on the metre, the kilogram and the second (following discussion within IEC and IUPAP).

1946 As a first step that had already been planned in 1933, the CIPM approved the MKS (metre, kilogram and second) System to replace the former system of electrical units called the “international System”.

1948 The 9th CGPM requested the CIPM to launch an international survey, the outcome of which was to be used to formulate recommendations for a single practical system of measurement units, suitable for adoption by all countries.

1954 The CGPM approved the introduction of the ampere, the kelvin and the candela as base units for electric current, thermodynamic temperature and luminous intensity respectively.

1960 The 11th CGPM adopted the name of the International System of Units (SI) for the system based on six base units: the metre, the kilogram, the second, the ampere, the kelvin and the candela. The 11th CGPM also adopted a new definition of the metre.

1967 The second was redefined as an “atomic second”. The new definition depended henceforth on the properties of a caesium atom.

1971 The 14th CGPM added a new unit to the SI: the mole as the unit for amount of substance.

1979 The candela was redefined in terms of a monochromatic radiation.

1983 For the first time a definition of a base unit of the SI was based on a fundamental constant: the speed of light. The metre was henceforth the length of the path travelled by light in vacuum during a specific fraction of a second.

1990 New practical conventions based on quantum phenomena were adopted for the ohm and the volt.

16 Nov 2018 Four base units of the SI will be redefined: each definition will be linked to a constant of physics. The 1990 conventions will no longer be needed and will be abolished.

20 May 2019 World Metrology Day on 20 May 2019 will mark the official entry into force of the revised SI if agreed at the 26th CGPM.

CGPM Press kit link: https://www.bipm.org/utils/en/pdf/CGPM/Press-kit.pdf