Metre

The metre (or meter) is the basic unit of length in the International System of Units (SI). Historically, the metre was defined by the French Academy of Sciences as the length between two marks on a platinum- iridium bar, which was designed to represent one ten-millionth of the distance from the Equator to the North Pole along the Paris Meridian. In 1983, the metre was redefined as the distance travelled by light in free space in of a second.The BIPM does not distinguish between quantum vacuum and free space. Resolution 1 of the 17th CGPM ( CGPM, 1984), retrieved from BIPM database (BIPM, n.d.) on 24 August 2008. The symbol for metre is m. Decimal multiples such as kilometre and centimetre are indicated by adding SI prefixes to metre.

History

Name

The first recorded proposal for a decimal-based unit of length was the universal measure unit proposed by the English philosopher John Wilkins in 1668. An Essay towards a Real Character and a Philosophical Language (Reproduction) An Essay towards a Real Character and a Philosophical Language (Transcription) In 1675 the Italian scientist Tito Livio Burattini, in his work Misura Universale, used the words metro cattolico ( ) which was derived from the Greek (métron katholikos), "a ". This word gave rise to the French mètre which in 1797 was introduced into the English language.meter. (2009). In Merriam-Webster Online Dictionary. Retrieved 8 December 2009.

Meridional definition

In 1668 Wilkins proposed using a pendulum that had a period of one second as the standard for a universal decimal-based unit of length . Such a pendulum would have resulted in the standard being 38 Rhineland or 39¼ English inches (997 mm)in length. In the eighteenth century, there were two favoured approaches to the definition of the standard unit of length. One approach suggested defining the metre as the length of a pendulum with a half- period of one second, a ' seconds pendulum'. The other approach suggested defining the metre as one ten-millionth of the length of the Earth's meridian along a quadrant, that is the distance from the Equator to the North Pole. In 1791, the French Academy of Sciences selected the meridional definition over the pendular definition because the force of gravity varies slightly over the surface of the Earth, which affects the period of a pendulum. In order to establish a universally accepted foundation for the definition of the metre, measurements of this meridian more accurate than those available at that time were imperative. The Bureau des Longitudes commissioned an expedition led by Delambre and Pierre Méchain, lasting from 1792 to 1799, which measured the length of the meridian between Dunkerque and Barcelona. This portion of the meridian, which also passes through Paris, was to serve as the basis for the length of the half meridian, connecting the North Pole with the Equator. (The exact shape of the Earth is discussed at Geoid.) However, in 1793, France adopted as its official unit of length a metre based on provisional results from the expedition. Although it was later determined that the first prototype metre bar was short by a fifth of a millimetre because of miscalculation of the flattening of the Earth, this length became the standard. The circumference of the Earth through the poles is therefore slightly more than forty million metres.

Prototype metre bar

In the 1870s and in light of modern precision, a series of international conferences were held to devise new metric standards. The Metre Convention (Convention du Mètre) of 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 organisation would preserve the new prototype metre and kilogram standards when constructed, distribute national metric prototypes, and maintain comparisons between them and non-metric measurement standards. The organisation created a new prototype bar in 1889 at the first General Conference on Weights and Measures (CGPM: Conférence Générale des Poids et Mesures), establishing the International Prototype Metre as the distance between two lines on a standard bar composed of an alloy of ninety percent platinum and ten percent iridium, measured at the melting point of ice. The original international prototype of the metre is still kept at the BIPM under the conditions specified in 1889. A discussion of measurements of a standard metre bar and the errors encountered in making the measurements is found in a NIST document.

Standard wavelength of krypton-86 emission

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 CGPM defined the metre in the new SI system as equal to 1,650,763.73 wavelengths of the orange- red emission line in the electromagnetic spectrum of the krypton-86 atom in a vacuum.

Standard wavelength of helium-neon laser light

To further reduce uncertainty, the seventeenth CGPM in 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: of a second.}} This definition effectively fixed the speed of light in a vacuum at precisely 299,792,458 metres per second. Although the metre is now defined in terms of time-of-flight, actual laboratory realisations of the metre are still delineated by counting the required number of wavelengths of light along the distance. Three major factors limit the accuracy attainable with laser interferometers: The term micron is often used instead of micrometre, but this practice is officially discouraged. NIST Guide to the SI: #5.2.3 Other Unacceptable Units - Retrieved 12 March 2010

Spelling

Two spellings of the name of the unit are common in English: metre is preferred among the majority of countries in the English-speaking world except in the United States, where the spelling is meter.See American and British English spelling differences#-re, -er The most recent official brochure, written in 2006, about the International System of Units (SI), Bureau international des poids et mesures, was written in French by the International Bureau of Weights and Measures. An English translation (using the spelling: metre) is included to make the SI standard "more widely accessible". BIPM, 2006, p. 130ff. In 2008, the U.S. English translation published by the U.S. National Institute of Standards and Technology chose to use meter in accordance with the United States Government Printing Office Style Manual.The Metric Conversion Act of 1975 gives the Secretary of Commerce of the US the responsibility of interpreting or modifying the SI for use in the US. The Secretary of Commerce delegated this authority to the Director of the National Institute of Standards and Technology (NIST) ( Turner). In 2008, NIST published the US version (Taylor and Thompson, 2008a) of the English text of the eighth edition of the BIPM publication Le Système international d'unités (SI) (BIPM, 2006). In the NIST publication, the spellings "meter," "liter," and "deka" are used rather than "metre", "litre", and "deca" as in the original BIPM English text (Taylor and Thompson, 2008a, p. iii). The Director of the NIST officially recognised this publication, together with Taylor and Thompson (2008b), as the "legal interpretation" of the SI for the United States (Turner). The spelling (-)meter for measuring devices, is the only correct spelling regardless of country, such as: parking meter, speedometer. Cambridge Advanced Learner's Dictionary (2008). Cambridge University Press. s.v. parking meter, meter, speedometer. (Meter, the device, has the same derivation as the metre detailed in this article.American Heritage Dictionary of the English Language. 3rd ed. (1992). Boston: Houghton Mifflin. s.v. meter.

Equivalents in other units

Within this table, "inch" means "international inch".A. V. Astin & H. Arnold Karo, (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, 8:45 a.m.) "≈" means "is approximately equal to". "≡" means "equals by definition" or equivalently, "is exactly equal to".

Notes

References

Bureau International des Poids et Mesures. (2006). The International System of Units (SI). Retrieved 18 August 2008.
* HTML version. Retrieved 24 August 2008.
Bureau International des Poids et Mesures. (n.d.). Resolutions of the CGPM (search facility). Retrieved 3 June 2006.
Bureau International des Poids et Mesures. (n.d.). The BIPM and the evolution of the definition of the metre. Retrieved 3 June 2006.
Layer, H.P. (2008). Length—Evolution from Measurement Standard to a Fundamental Constant. Gaithersburg, MD: National Institute of Standards and Technology. Retrieved 18 August 2008.
Mohr, P., Taylor, B.N., and David B. Newell, D. (28 December 2007). CODATA Recommended Values of the Fundamental Physical Constants: 2006. Gaithersburg, MD: National Institute of Standards and Technology. Retrieved 18 August 2008.
National Institute of Standards and Technology. (December 2003). The NIST Reference on Constants, Units, and Uncertainty: International System of Units (SI) (web site):
* SI base units. Retrieved 18 August 2008.
* Definitions of the SI base units. Retrieved 18 August 2008.
* Historical context of the SI: Metre. Retrieved 18 August 2008.
National Research Council Canada. (16 May 2008). Optical Frequency - Maintaining the SI Metre. Retrieved 18 August 2008.
Penzes, W. (29 December 2005). Time Line for the Definition of the Meter. Gaithersburg, MD: National Institute of Standards and Technology — Precision Engineering Division. Retrieved 3 June 2006.
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.
Tibo Qorl. (2005) The History of the Meter (Translated by Sibille Rouzaud). Retrieved 18 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. 28432-3.

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