Metric system
The National Convention enacts the law of 18 Germinal establishing France’s decimal metric system
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Original prompt & settings (JSON) ↗France legislated a provisional platinum metre because the two astronomers measuring the Earth had not yet finished their survey.
18 Germinal, Year III
On 18 Germinal Year III — 7 April 1795 — the French National Convention enacted a comprehensive law governing the decimal metric system and its nomenclature.
The law fixed five names that are still immediately recognizable: metre, are, stère, litre and gramme, together with decimal multiples and submultiples formed with prefixes.
It also created a temporary weights-and-measures agency to supervise the replacement of the older measures.
A standard legislated before it could be measured
The metre was meant to be one ten-millionth of the meridian quadrant running from the equator to the North Pole. Jean-Baptiste Delambre and Pierre Méchain were still out measuring the Dunkirk–Barcelona arc needed to calculate it.
So the law called for a provisional platinum metre in the meantime. The definitive platinum Metre and Kilogram of the Archives were deposited in 1799.
The original kilogram standard was platinum; the platinum-iridium International Prototype Kilogram belongs to a later standard of 1889.
Not the first adoption, and not the day France went metric
Calling this the first adoption skips the Convention's own decree of 1 August 1793, which introduced the first legal version of the system. The 1793 legislation had been hurried, and contained omissions and errors the new law was written to settle.
Passing a law was much faster than changing daily life. Prototype standards still had to be manufactured, copied and distributed, and everyday measures did not change everywhere at once.
Exclusive metric use did not become compulsory throughout France until 1 January 1840.
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