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:''For other uses of "metre" and "meter", see [[Metre (disambiguation)]].'' |
:''For other uses of "metre" and "meter", see [[Metre (disambiguation)]].'' |
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The '''metre''' is the [[SI base unit| |
The '''metre''' is the [[SI base unit|basic unit]] of [[length]] in the [[International System of Units]] (SI: Systéme International d'Unités). It is defined as the [[length]] of the path travelled by [[light]] in [[vacuum]] during a [[time]] interval of 1/299,792,458 of a [[second]]. The symbol of the metre is '''m'''. Metre is also spelled '''meter''' in [[American English]]. |
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The metre is defined as the length of path travelled by [[light]] in [[vacuum]] during a time interval of 1/299,792,458 of a [[second]]. |
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For perspectives of the length of a metre, see [[1 E0 m]]. For comparisons with other units, see [[conversion of units]]. |
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The symbol of the metre in the SI is '''m'''. |
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Metre is spelled '''meter''' in [[American English]]. |
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See [[1 E0 m]] for comparisons of the length of a metre. |
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See also [[conversion of units]]. |
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== Multiples == |
== Multiples == |
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[[SI prefix]]es are used to name multiples and subdivisions of the metre |
[[SI prefix]]es are used to name multiples and subdivisions of the metre: |
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: [[terametre]] = [[1 E12 m|10<sup>12</sup> m]] (rare) |
: [[terametre]] = [[1 E12 m|10<sup>12</sup> m]] (rare) |
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: [[picometre]] = [[1 E-12 m|10<sup>-12</sup> of a metre]] |
: [[picometre]] = [[1 E-12 m|10<sup>-12</sup> of a metre]] |
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: [[femtometre]] = [[1 E-15 m|10<sup>-15</sup> of a metre]] |
: [[femtometre]] = [[1 E-15 m|10<sup>-15</sup> of a metre]] |
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== History == |
== History == |
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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 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 millimeter due to miscalculation of the flattening of the earth, this length became the standard. |
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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 (element)|iridium]], called the International Prototype Meter, kept at [[Sevres]]. |
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In the 1870's and in light of modern precision, a series of international conferences were held to devise new metric standards. The [[Treaty of the Meter]] (1875) mandated the establishment of a permanent [[International Bureau of Weights and Measures]] (BIPM: Bureau International des Poids et Mesures) to be located in [[Sevres]], 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 <i>International Prototype Metre</i> as the distance between two lines on a standard alloyed bar of ninety percent [[platinum]] and ten percent [[iridium]]. |
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In 1893, the standard metre was first measured with an [[interferometer]] by [[Albert Abraham Michelson|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 [[International System of Units|SI]] system as equal to 1,650,763.73 wavelengths of the [[orange (colour)|orange]]-[[red]] [[emission line]] in the [[electromagnetic spectrum|spectrum]] of the [[krypton]]-86 [[atom]] in a [[vacuum]]. |
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In [[1892]], [[Albert Abraham Michelson|A.A. Michelson]] used an [[interferometer]] which he devised to measure the length of the International Prototype Meter. |
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By [[1925]] interferometry was in regular use at the International Bureau of Weights and Measures to measure length. |
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However, the International Prototype Meter remained the primary standard from 1889 until 1960. |
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To further reduce uncertainty, the seventeenth CGPM of 1983 replaced the definition of the metre with its current definition, in terms of the [[speed of light]]. Note that this exactly fixes the speed of light in a vacuum at 299,792,458 meters per second. Definitions based on the physical properties of light are more reproducible and precise because the properties of light are considered to be universally constant. |
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In [[1960]], 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 (colour)|orange]]-[[red]] [[emission line]] in the [[electromagnetic spectrum|spectrum]] of [[krypton]]-86. |
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With the growing availability of [[laser]]s, |
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by [[1964]] [[helium]]-[[neon]] lasers were commonly used to measure length. |
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Although the primary definition was still based on krypton-86, |
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helium-neon lasers were found useful due to the [[coherence]] of their emitted light, |
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which made it possible to measure long distances very accurately. |
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The original international prototype of the metre is still kept at the BIPM under the conditions specified in 1889. |
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In [[1983]] the General Conference on Weights and Measures defined the metre as the distance travelled by light in a vacuum in 1/299,792,458 of a [[second]]. |
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That is, the [[speed of light]] in a vacuum was defined to be exactly 299,792,458 metres per second. |
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Compared to the 1960 definition, the 1983 definition has a subtle change of emphasis: |
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before, |
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the length of the metre was given an arbitrary definition, |
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and the speed of light was measured in terms of the metre. |
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However, |
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the 1983 definition defines the metre in terms of the speed of light. |
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Definitions based on the physical properties of light are easier to maintain and more consistent than a measurement based on the circumference of the Earth or the length of a specific metal bar, |
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since the properties of light are believed to be the same everywhere. |
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Thus, should the prototype metre be destroyed or lost, the standard metre can still be easily recreated in a suitably equipped laboratory. |
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The current standard |
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also has the advantage that it can be measured with far greater precision than the circumference of the earth or the distance between two lines. |
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== External links== |
== External links== |
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*[http://www.sengpielaudio.com/ConvLeng.htm Conversions of English and American length units to metric units] |
*[http://www.sengpielaudio.com/ConvLeng.htm Conversions of English and American length units to metric units] |
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*[http://www.ex.ac.uk/trol/scol/ccleng.htm Conversion calculator for units of length (Cleave Books)] |
*[http://www.ex.ac.uk/trol/scol/ccleng.htm Conversion calculator for units of length (Cleave Books)] |
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* [http:// |
* [http://physics.nist.gov/cuu/Units/meter.html History of the Meter at the U.S. National Institute of Standards and Technology (NIST)] |
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* [http://www.mel.nist.gov/div821/museum/timeline.htm Timeline of the History Meter at the NIST] |
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[[ca:Metre]] [[cs:metr]] [[da:Meter]] [[de:Meter]] [[eo:Metro]] [[es:Metro]] [[et:Meeter]] [[fi:Metri]] [[fr:Mètre]] [[gl:Metro]] [[ia:Metro]] [[id:Meter]] [[it:Metro]] [[ja:メートル]] [[ko:미터]] [[ms:Meter]] [[nl:Meter]] [[pl:Metr]] [[pt:Metro]] [[ro:Metru]] [[simple:Metre]] [[sl:meter]] [[uk:Метр]] [[zh:公尺]] [[ru:Метр]] |
[[ca:Metre]] [[cs:metr]] [[da:Meter]] [[de:Meter]] [[eo:Metro]] [[es:Metro]] [[et:Meeter]] [[fi:Metri]] [[fr:Mètre]] [[gl:Metro]] [[ia:Metro]] [[id:Meter]] [[it:Metro]] [[ja:メートル]] [[ko:미터]] [[ms:Meter]] [[nl:Meter]] [[pl:Metr]] [[pt:Metro]] [[ro:Metru]] [[simple:Metre]] [[sl:meter]] [[uk:Метр]] [[zh:公尺]] [[ru:Метр]] |
Revision as of 20:45, 15 May 2004
- 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: Systéme International d'Unités). It is defined as the length of the path travelled by light in vacuum during a time interval of 1/299,792,458 of a second. The symbol of the metre is m. Metre is also spelled meter in American English.
For perspectives of the length of a metre, see 1 E0 m. For comparisons with other units, see conversion of units.
Multiples
SI prefixes are used to name multiples and subdivisions of the metre:
- terametre = 1012 m (rare)
- gigametre = 109 m (rare)
- megametre = 106 m (rare)
- kilometre = 1,000 metres
- hectometre = 100 metres (rare)
- decametre = 10 metres (rare)
- decimetre = 1/10 metre
- centimetre = 1/100 metre
- millimetre = 1/1000 metre
- micrometre (formerly micron) = 1 millionth of a metre
- nanometre = 10-9 of a metre
- picometre = 10-12 of a metre
- femtometre = 10-15 of a metre
History
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 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 millimeter due to miscalculation of the flattening of the earth, this length became the standard.
In the 1870's and in light of modern precision, a series of international conferences were held to devise new metric standards. The Treaty of the Meter (1875) mandated the establishment of a permanent International Bureau of Weights and Measures (BIPM: Bureau International des Poids et Mesures) to be located in Sevres, 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 alloyed bar 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, in terms of the speed of light. Note that this exactly fixes the speed of light in a vacuum at 299,792,458 meters per second. Definitions based on the physical properties of light are more reproducible and precise 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.
External links
- Length conversion: millimeters and micrometers to meters and prefixes
- Conversions of English and American length units to metric units
- Conversion calculator for units of length (Cleave Books)
- History of the Meter at the U.S. National Institute of Standards and Technology (NIST)
- Timeline of the History Meter at the NIST