- For other uses of "metre" and "meter", see Metre (disambiguation).
The metre is the basic unit of length in the International System of Units (SI: Système International d'Unités). It is defined as the length of the path travelled by light in absolute vacuum during a time interval of 1/299,792,458 of a second. It is equal to 10000/254 inches, approximately 39.37 inches. The symbol of the metre is m. Metre is also spelled meter in American English.
millimetre << centimetre << decimetre << metre << decametre << hectometre << kilometre
The word itself is from the Greek metron (μετρον), "a measure" via the French mètre. Its first recorded usage in English is from 1797.
In the eighteenth century, there were two favored approaches to the definition of the standard unit of length. One suggested defining the metre as the length of a pendulum with a half-period of one second. The other suggested defining the metre as one ten-millionth of the length of the earth's meridian along a quadrant (one-fourth the polar circumference of the earth). In 1791, the French Academy of Sciences selected the meridional definition, using the meridian of Paris, over the pendular definition because of the slight variation of the force of gravity over the surface of the earth, which affects the period of a pendulum. In 1795, France adopted the metre as its official unit of length. Although the first prototype metre bar was short by a fifth of a millimetre due to miscalculation of the flattening of the earth, this length became the standard. So, the circumference of the Earth through the poles is approximately forty million metres.
In the 1870s and in light of modern precision, a series of international conferences were held to devise new metric standards. The Treaty of the Metre (1875) mandated the establishment of a permanent International Bureau of Weights and Measures (BIPM: Bureau International des Poids et Mesures) to be located in Sèvres, France. This new organization would preserve the new prototype metre and kilogram when constructed, and would maintain comparisons between them and the basic units of other, nonmetric, weights and measures. This organization created a new prototype bar in 1889, establishing the International Prototype Metre as the distance between two lines on a standard bar of an alloy of ninety percent platinum and ten percent iridium.
In 1893, the standard metre was first measured with an interferometer by Albert A. Michelson, the inventor of the device and an advocate of using some particular wavelength of light as a standard of distance. By 1925, interferometry was in regular use at the BIPM. However, the International Prototype Metre remained the standard until 1960, when the eleventh General Conference on Weights and Measures (CGPM: Conférence Générale des Poids et Mesures) defined the metre in the new SI system as equal to 1,650,763.73 wavelengths of the orange-red emission line in the spectrum of the krypton-86 atom in a vacuum.
To further reduce uncertainty, the seventeenth CGPM of 1983 replaced the definition of the metre with its current definition, thus fixing the length of the metre in terms of time and the speed of light:
- The metre is the length of the path travelled by light in vacuum during a time interval of 1/299 792 458 of a second.
Note that this definition exactly fixes the speed of light in a vacuum at 299,792,458 metres per second. Definitions based on the physical properties of light are more precise and reproducible because the properties of light are considered to be universally constant.
The original international prototype of the metre is still kept at the BIPM under the conditions specified in 1889.
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