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# Metre

The metre (American spelling: meter, symbol: m) is the basic unit of distance (or of "length", in the parlance of the physical sciences) in the International System of Units. It is defined as the length of the path travelled by light in an absolute vacuum during a time interval of exactly 1/299,792,458 of a second. This definition does not change the size of the unit (see History below), but it was introduced to take into account recent developments in measurement techniques whereby length and time can be reproduced with very high accuracy — in the case of time, to an accuracy of 1013. One metre is equal to 10000/254 inches, approximately 39.37 inches.

millimetre << centimetre << decimetre << metre << decametre << hectometre << kilometre

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## History

The means of defining the metre has changed over time:

• 1793: 1/10,000,000 of the distance from the pole to the equator.
• 1795: Provisional metre bar constructed in brass.
• 1799: Definitive prototype metre bars constructed in platinum.
• 1889: International prototype metre bar in platinum-iridium, cross-section X.
• 1960: Krypton spectrum: 1650763.73 wavelengths in vacuum of the radiation corresponding to the transition between levels 2p10 and 5d5 of the krypton-86 atom.
• 1983: Speed of light definiton: Length traveled by light in vacuum during 1/299792458 of a second.

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 favoured 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 August 1793, the Republican Government in France decreed that the standard unit of length would be 10 - 7 of the earth's quadrant passing through Paris and that the unit be called the metre. Five years later the survey of the arc was completed and three platinum standards and several iron copies were made. Subsequent analysis showed that the length of the earth's quadrant had been incorrectly surveyed resulting in the first prototype metre bar being short by a fifth of a millimetre (due to miscalculation of the flattening of the earth), instead of altering the length of the metre to maintain the 10 - 7 ratio, the metre was redefined as the distance between two marks on a bar. So, the circumference of the Earth through the poles is only 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 national standards for the metre and the kilogram. This organisation created a new prototype bar in 1889, establishing the International Prototype Metre as the distance between two lines on its standard-issue 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. The original international prototype of the metre is still kept at the BIPM under the conditions specified in 1889.

The eleventh General Conference on Weights and Measures (CGPM: Conférence Générale des Poids et Mesures) in 1960 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.

## References

• A Dictionary of Scientific Units - including dimensionless numbers and scales. 5th Edition 1986. H.G. Jerrard and D.B. McNeill.