Sevres.science / The SI

From the metre to the constants

1791–1799: a measure from the Earth

In 1791 the French Assembly took a quarter of the Earth’s meridian as the basis of the new system: the metre was to be one ten-millionth of it. Between 1792 and 1798 Jean-Baptiste Delambre and Pierre Méchain measured an arc of the Paris meridian from Dunkirk to Barcelona, and a law of 10 December 1799 defined the metre.

1796–1799: metres in the street

To teach Parisians the new measure, the authorities ordered sixteen marble standard metres in 1795, set into walls around the city between 1796 and 1799. Two survive in Paris: one at 36 rue de Vaugirard, the only one still in its original place, and one on the façade of the Ministry of Justice on Place Vendôme, restored and reinstalled in 2021.

1875: the Metre Convention

On 20 May 1875 seventeen signatories concluded the Metre Convention, which set up the International Bureau of Weights and Measures. France granted the Bureau the Pavillon de Breteuil in Sèvres, where it still works today. The date is now celebrated every year as World Metrology Day.

1889: the prototypes

The first General Conference on Weights and Measures, in 1889, sanctioned the international prototypes of the metre and the kilogram, both made of platinum with 10 % iridium. The metre was the distance between two marks on the bar at the temperature of melting ice; the kilogram was the mass of a cylinder about 39 mm high and wide, kept at the Bureau ever since.

Poland joined the Metre Convention in 1925.

1948–1954: electricity and six base units

In 1948 the 9th General Conference ratified definitions of the electrical units, among them an ampere defined by the force between two parallel conductors carrying a current. In 1954 the 10th General Conference adopted six base units for a practical system: the metre, kilogram, second, ampere, degree Kelvin and candela.

1960–1983: the SI takes shape

In 1960 the 11th General Conference gave the system its name, Système international d’unités, with the abbreviation SI in every language. It also retired the platinum-iridium metre bar: the metre became 1 650 763.73 wavelengths of a particular orange line of krypton-86. In 1967–1968 the second was redefined by the caesium atom and the “degree Kelvin” became simply the kelvin, symbol K. In 1971 the mole joined the base units, in 1979 the candela received its present definition and in 1983 the metre was defined by the speed of light.

1999: mutual recognition

In 1999 the CIPM drew up the Mutual Recognition Arrangement, under which national metrology institutes recognise one another’s measurement standards and calibration certificates on the basis of published key comparisons.

2018–2019: units from constants

In November 2018 the 26th General Conference fixed the values of four more constants and withdrew the old definitions. The new SI came into force on 20 May 2019. The kilogram stopped being an object, and all seven base units came to rest on constants of nature.

2022: new prefixes

In November 2022 the 27th General Conference added four prefixes, ronna, quetta, ronto and quecto, the first new ones in three decades.

2026: continuous UTC, and the second next

In 2026 the 28th General Conference decided that UTC will run without leap seconds from 20 May 2027 and asked for a proposal for a new definition of the second to be presented in 2030. Like the redefinition of 2019, it is meant to change nothing in everyday life.