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Math and Arithmetic

Who keeps track of the length of a meter?


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July 08, 2015 8:05PM

In 1791 the French Academy of Science defined the metre as one ten millionths of the distance from the North Pole to the equator along the meridian through Paris.

In 1875 the Metre Convention agreed to set up a permanent International Bureau of Weights and Measures (BIPM, after its French acronym) in Sèvres, France. Here, the international metre was the distance between two lines on a standard bar composed of an alloy of 90% platinum and 10% iridium, measured at the melting point of ice. Copies of the prototype were sent to national standards organisations in other nations.

Although it was later discovered that because of the shape of the earth this measure was not quite one ten millionths of the pole-to-equator distance, the definition remained.

In 1960, a metre was more finely defined as 1 650 763.73 wavelengths of the orange-red emission line in the electromagnetic spectrum of the krypton-86 atom in a vacuum.

In 1983 the metre was redefined in terms of the second and the speed of light. It is the path travelled by light in vacuum during a time interval of 1/299792458 of a second.

The organisation responsible for this definition (and previous versions) is the General Conference on Weights and Measures (CGPM: Conférence Générale des Poids et Mesures).