# Metre

*This page is about the metre as a unit of measurement. For other uses see Metre (disambiguation)*

The **metre** (symbol: m, spelled **meter** in American English) is the SI base unit of length. It is defined as the length of path traveled by light in vacuum during a time interval of 1/299,792,458 of a second. See 1 E0 m for comparisons of the length of a metre.

SI prefixes are used to name multiples and subdivisions of the metre. The most commonly used ones are:

- kilometre = 1,000 metres
- decametre = 10 metres (rare)
- centimetre = 1/100 metre
- millimetre = 1/1000 metre
- micrometre (formerly
*micron*) = 1 millionth of a metre
- nanometre = 1 billionth of a metre
- picometre = 10
^{-12} of a metre
- femtometre = 10
^{-15} of a metre

## History

The metre was originally defined in 1791 by the French Academy of Sciences as 1/10,000,000 of the distance along the Earth's surface from the North Pole to the Equator along the meridian of Paris and on April 7, 1795 France adopted the metre as its official unit of length. Uncertainty in the measurement of that distance led the International Bureau of Weights and Measures in 1889 to redefine the metre as the distance between two lines on a standard bar of platinum-iridium kept at Sevres.

In 1960, as lasers had become available, the 11th General Conference on Weights and Measures changed the definition of metre to be the length of 1,650,763.73 wavelengths in vacuum of the orange-red emission line in the spectrum of krypton-86. In 1983 the General Conference on Weights and Measures defined the metre as the distance traveled by light in a vacuum in 1/299,792,458 of a second (that is, the speed of light in a vacuum was defined to be 299,792,458 metres per second). Since the speed of light in vacuum is believed to be the same everywhere, this definition is easier to maintain and more consistent than a measurement based on the circumference of the Earth or the length of a specific metal bar. Thus, should the bar be destroyed or lost, the standard meter can still be easily recreated in any laboratory. It also has the advantage that it can (at least in theory) be measured with far greater precision than the circumference of the earth or the distance between two lines.

See also: SI, conversion of units