Measured in
Measuring distance as
Alvaro Deibe says:
The meter, or metre, (metro in Spain) was born in 1670 in France (abate Mouton, bishop of Lyon). He suggest using the length of an meridian arc equivalent to a minute of angle (don't know if my english will be up to the task). This unit was called "mile", and its fractions "virga" and "feet" (1/1000 and 1/10000 of a mile, respectively). The length of the meridian was (is) not so simple to compute, so Mouton showed how one could calculate these measures from a pendulum with a 1 second period.In 1671 Picard suggested using the length of the pendulum as the reference of lengths. But the period of the pendulum varies with latitude, so in 1745 the French Science Academy proposes using the length of the 1-second-period pendulum in the Equator as the measure basis.
Then, in 1790, with the French Revolution, Le Prieur proposes changing to the length of the 1-second pendulum IN PARIS (yeah, really!). With this measure in mind, a rule of Platinum-Iridium is forged, and stored in the Louvre Museum. This become the meter standard (at 10ºC, of course) for a long time. One third of this length was called feet, and the feet was made up of 10 inches, with 10 "lines" in each.
In 1790 Talleyrand (again, bishop in Autun, France) proposes finding an international solution to the length measure, and makes a "joint venture" between the National Assembly and Sciences Academy (France) and the Parlament and Royal Society (UK). The meter is defined in the length of the 1-second pendulum in a 45º latitude.
The problems with the latitude dependence of this definition of the meter finally ended in 1792, when Borda, Lagrange, Monge and Condorcet suggested using 1/4 of the length of an earth meridian. The meter was then defined as 10e-7 of this length.
The internationalization of the length measure grows, French, English, North American (contacts made with President Jefferson) and Spanish (Gabriel de Ciscar, a scientific sended to Paris) try to measure the 1/4 of the meridian 45. Finally, it was measured only a fraction of the meridian 45, between Dunquerque (France) and Barcelona (Spain) both of them at sea level.
From here, the definition of the meter changed as the science needed more resolution, precision, repeatibility and stability than that of the platinum-iridium in the Louvre museum. Definitions based in the speed of Light, the wavelength of a red mark in the atomic spectrum of Cadmium (1921), wavelengths of the radiation from a 86Kripton isotope in transition between 2p10 and 5ds status(1960), the wavelength of "saturated absorption"
Lasers (1970) and, finally, the distance travelled by light in 1/299.792.458 seconds... (the circle finally closes again: the space measure is derived from the time measure, just like in the 1-second pendulum).
The bottom line here is that the meter (and other units) evolved from a proprietary point of view to a more democratic measure, based on a natural property, that can be measured everywhere, in the Earth and out of it.
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