In the midst of the French Revolution, the National Assembly decides to create a decimal system of measurement. The metric system is born.
The first meter was based on clock-making: the length of a pendulum with a half-period (one-way swing) of one second. Responding to a proposal by the French Academy of Sciences, the assembly redefined the meter, in 1793, as 1/10,000,000 of the distance from the equator to the North Pole. The system was elegant. All conversions were based on 10, with Greek prefixes (deka-, hecto-, kilo-) for multiples, and Latin (deci-, centi-, milli-) for fractions. The gram unit of weight was defined by the weight of one cubic centimeter (aka one milliliter) of water.
The new Republican measures became legal throughout France in 1795 and were made compulsory in 1799, when definitive platinum meter bars and kilogram weights were constructed. But resistance to the new measures lasted for decades.
France also used a quasi-metric revolutionary calendar with months consisting of three décades of ten days each (see here).
The current International System of Units—or SI, for Système International—is based on the Treaty of the Meter signed in Paris on May 20, 1875. The United States was a signatory, and the metric system is the legal system in this country, although the legal alternate English system remains more widely used.
The meter was formally redefined in 1960 as 1,650,763.73 wavelengths in a vacuum of the orange-red light radiation of the krypton 86 atom (see here). The new standard was a hundred times more precise than the old. The current definition, adopted in 1983, makes the meter the distance traveled by light in a vacuum during 1/299,792,458 of a second.
That’s 39.37 inches to counterrevolutionaries.—RA