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Symbol m. The SI unit of length, being the length of the path travelled by light in vacuum during a time interval of 1/(2.99 792 458×10 8) second. This definition, adopted by the General Conference on Weights and Measures in October, 1983, replaced the 1967 definition based on the krypton lamp, i.e. 1 650 763.73 wavelengths in a vacuum of the radiation corresponding to the transition between the levels 2p 10 and 5d 5 of the nuclide krypton–86. This definition (in 1958) replaced the older definition of a metre based on a platinum-iridium bar of standard length. When the metric system was introduced in 1791 in France, the metre was intended to be one ten-millionth of the earth's meridian quadrant passing through Paris. However, the original geodetic surveys proved the impractibility of such a standard and the original platinum metre bar, the mètre des archives, was constructed in 1793.

Subjects: Chemistry.

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