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.