10 Dec 1799 France adopts the metric system, first country to do so

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The metric system, that is the system of units based on the metre, was officially adopted in France on 10 December 1799 (19 frimaire An VIII) and became the sole legal system of weights and measures there from 1801.

In France, before the start of the Revolution in 1789, there had been no uniformity of weights and measures. Trading had been difficult and fraud had been easy, so in 1790 the French National Assembly called for uniform new measures to put a stop to the abuses taking place. Charles Maurice Talleyrand, Bishop of Autun, presented to the Assembly a scheme based upon “natural” measures which he proudly stated would be “for all people, for all time”. This was modified by the Academy of Science, which also strongly favoured the new measures forming a decimal system as follows. The unit of length, the metre, was to be equal to the ten-millionth part of the arc of meridian between the North Pole and the equator and passing through Paris. The gram was to be the “absolute weight” of a volume of pure water, equal to the cube of the hundredth part of the metre and at the temperature of melting ice.

The length of the quarter meridian was calculated from measurements made between Dunkirk and Barcelona. The survey was by two astronomers, Jean-Baptiste Delambre and Pierre Méchain who, because of the troubled state of the country, narrowly missed losing their heads on a number of occasions after being arrested as spies. Long before this work was completed the measurement of length, area and volume based on the metre and mass (weight) based on the gram had been made obligatory by a law of 1793, with an approximate value for the metre.

A law of 10 December 1799 specified that one particular bar was to be the definitive standard of length throughout the Republic of France, and one cylinder the definitive standard of weight; these later became known as the Kilogramme des Archives and the Metre des Archives. The French did not take happily to the introduction of the metric weights and measures so in 1812 Emperor Napoleon allowed again the use of old names of measures but now with fixed new values in terms of the metre or kilogram (1000 grams). A further law of 1837 made the use of metric measures obligatory again from 1 January 1840. Elsewhere in Europe, The Netherlands had been truly metric from 1820 and Greece from 1836.

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