meter (m)

The base unit of length in the SI system of units. 1 meter = 39.37 inches. It is defined by the International Bureau of Weights and Measures as the distance traveled by light in a vacuum in 1/299,792,458 of a second. It was originally intended that the meter represent one ten millionth of the distance from the North Pole to the equator on the meridian passing through Paris. But the surveyors got their sums wrong and for 162 years (to 1960), the meter was defined as an arbitrary distance marked on a metal bar, the "international prototype metre". From 1960 to 1983, the meter was defined as the length equal to 1,650,763.73 times the wavelength of radiation corresponding to the transition between the energy levels 2p₁₀ and 5d₅ of the krypton-86 atom.

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• UNITS

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