Metre

The metre (or meter in US spelling; symbol: m) is the base unit of length in the International System of Units (SI). Since 2019, the metre has been defined as the length of the path travelled by light in vacuum during a time interval of of a second, where the second is defined by a hyperfine transition frequency of caesium.

The metre was originally defined in 1791 by the French National Assembly as one ten-millionth of the distance from the equator to the North Pole along a great circle, so the Earth's polar circumference is approximately .

In 1799, the metre was redefined in terms of a prototype metre bar. The bar used was changed in 1889, and in 1960 the metre was redefined in terms of a certain number of wavelengths of a certain emission line of krypton-86. The current definition was adopted in 1983 and modified slightly in 2002 to clarify that the metre is a measure of proper length. From 1983 until 2019, the metre was formally defined as the length of the path travelled by light in vacuum in of a second. After the 2019 revision of the SI, this definition was rephrased to include the definition of a second in terms of the caesium frequency . This series of amendments did not alter the size of the metre significantly – today Earth's polar circumference measures , a change of about 200 parts per million from the original value of exactly , which also includes improvements in the accuracy of measuring the circumference.

Spelling

''Metre'' is the standard spelling of the metric unit for length in nearly all English-speaking nations, the exceptions being the United States and the Philippines which use ''meter''.

Measuring devices (such as ammeter, speedometer) are spelled "-meter" in all variants of English. The suffix "-meter" has the same Greek origin as the unit of length.

Etymology

The etymological roots of ''metre'' can be traced to the Greek verb () ((I) measure, count or compare) and noun () (a measure), which were used for physical measurement, for poetic metre and by extension for moderation or avoiding extremism (as in "be measured in your response"). This range of uses is also found in Latin (), French (), English and other languages. The Greek word is derived from the Proto-Indo-European root '' *meh₁-'' 'to measure'. The motto () in the seal of the International Bureau of Weights and Measures (BIPM), was approved by Adolphe Hirsch on 11 July 1875 and may be translated as "Keep the measure", thus calls for both measurement and moderation. The use of the word ''metre'' (for the French unit ) in English began at least as early as 1797.Oxford English Dictionary, Clarendon Press 2nd ed. 1989, vol. IX p. 697 col. 3.

History of definition

SI prefixed forms of metre

SI prefixes can be used to denote decimal multiples and submultiples of the metre, as shown in the table below. Long distances are usually expressed in km, astronomical units (149.6 Gm), light-years (10 Pm), or parsecs (31 Pm), rather than in Mm or larger multiples; "30 cm", "30 m", and "300 m" are more common than "3 dm", "3 dam", and "3 hm", respectively.

The terms '' micron'' and '' millimicron'' have been used instead of ''micrometre'' (μm) and ''nanometre'' (nm), respectively, but this practice is discouraged.

Equivalents in other units

Within this table, "inch" and "yard" mean "international inch" and "international yard" respectively, though approximate conversions in the left column hold for both international and survey units.
: "≈" means "is approximately equal to";
: "=" means "is exactly equal to".

One metre is exactly equivalent to  inches and to  yards.

A simple mnemonic to assist with conversion is "three 3s": 1 metre is nearly equivalent to 3  feet  inches. This gives an overestimate of 0.125 mm.

The ancient Egyptian cubit was about 0.5 m (surviving rods are 523–529 mm). Scottish and English definitions of the ell (2 cubits) were 941 mm (0.941 m) and 1143 mm (1.143 m) respectively. The ancient Parisian ''toise'' (fathom) was slightly shorter than 2 m and was standardised at exactly 2 m in the mesures usuelles system, such that 1 m was exactly  toise. The Russian verst was 1.0668 km. The Swedish mil was 10.688 km, but was changed to 10 km when Sweden converted to metric units.

See also

* ISO 1standard reference temperature for length measurements
* Metric prefix
* Metrication

References

Cited bibliography

*
* Astin, A. V. & Karo, H. Arnold, (1959)
''Refinement of values for the yard and the pound''
Washington DC: National Bureau of Standards, republished on National Geodetic Survey web site and the Federal Register (Doc. 59–5442, Filed, 30 June 1959)
*
*
*
*
*
*


Retrieved 26 May 2010.
* National Institute of Standards and Technology. (27 June 2011).
NIST-F1 Cesium Fountain Atomic Clock
'. Author.
* National Physical Laboratory. (25 March 2010).
Iodine-Stabilised Lasers
'. Author.
*
* Republic of the Philippines. (2 December 1978).

'. Author.
* Republic of the Philippines. (10 October 1991). '' ttps://www.officialgazette.gov.ph/downloads/1991/10oct/19911010-RA-7160-CCA.pdf Republic Act No. 7160: The Local Government Code of the Philippines'. Author.
* Supreme Court of the Philippines (Second Division). (20 January 2010).
G.R. No. 185240
'. Author.
* Taylor, B.N. and Thompson, A. (Eds.). (2008a)
''The International System of Units (SI)''
United States version of the English text of the eighth edition (2006) of the International Bureau of Weights and Measures publication ''Le Système International d' Unités (SI)'' (Special Publication 330). Gaithersburg, MD: National Institute of Standards and Technology. Retrieved 18 August 2008.
* Taylor, B.N. and Thompson, A. (2008b)
''Guide for the Use of the International System of Units''
(Special Publication 811). Gaithersburg, MD: National Institute of Standards and Technology. Retrieved 23 August 2008.
* Turner, J. (deputy director of the National Institute of Standards and Technology). (16 May 2008)
"Interpretation of the International System of Units (the Metric System of Measurement) for the United States"
''Federal Register'' Vol. 73, No. 96, p.28432–28433.
* Zagar, B.G. (1999)
Laser interferometer displacement sensors
in J.G. Webster (ed.). ''The Measurement, Instrumentation, and Sensors Handbook''. CRC Press. .

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SI base units