# Metre (unit)

The metre (American meter) is the basic unit of length measurement in the International System of Units (SI) commonly known as the metric system. The metre is abbreviated as m, appearing after the quantity. The metre was initially adopted as a unit of measure in France in 1790, during the French Revolution.

The original definition of the metre was the length of a pendulum with a half-period of 1 second, but was changed in 1791 to be the length of a prototype bar which was supposed to be 1/10 000 000 of the length of the meridian of Paris from the north pole to the equator. Since then, the metre has been redefined a number of times, and the current SI definition now defines the metre in terms of the speed of light and the second, such that the metre is the length travelled by light in a vacuum in 1/299 792 458 of a second[1] Clearly, the practicality of adoption of a time of flight as a measure of length depends critically upon the experimental fact that the speed of light is a universal constant in vacuum, conveniently found to be a medium in which propagation is isotropic, non-dichroic, and both field-strength and frequency independent to within experimental error.[2] These characteristics mean comparisons of measurements are subject to few uncertainties, a major goal of metrology.

The metre is also the basis of the SI units of area and volume, which are the square metre (m2) and cubic metre (m3), respectively.

Very few adult humans are more than 2m in height.

The foot has been defined in the United States to equal exactly 0.3048m, though an earlier definition of the survey foot was 1200/3937 m, which is different than the current definition by about one part in 500 000. The inch, being 1/12 foot, is exactly 0.0254m, or 2.54cm.

The modern Chinese chǐ (市尺), or "Chinese foot", has been defined to equal exactly one-third of a meter. The Hong Kong chek (尺) is exactly 0.371475m. The Japanese kanejaku (曲尺) was defined as 10/33m in 1891.

The Spanish vara was fixed at about 835.9mm in 1801; however, the vara was defined in California as 838.2mm (33 inches), and in Texas as 846.666...mm (33 1/3 inches).

The megalithic yard is approximately 0.83m.

##  References

1. International Bureau of Weights and Measures From the website of the International Bureau of Weights and Measures.
2. Walter Dittrich, Holger Gies (2000). Probing the quantum vacuum: perturbative effective action approach in quantum electrodynamics and its application. Springer, pp. 1 ff. ISBN 3540674284.
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