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:''This article is about the unit of length. For other uses of ''metre'' or ''meter'', see '']''.''
{{Unit of length
|name=metre
|m=1
|accuracy=3 <!--Number of decimal places-->
}}
The '''metre''', or '''meter''' (]), is a measure of ]. It is the ] of ] in the ] and in the ] (SI), used around the world for general and scientific purposes. Historically, the metre was defined by the ] as 1/10,000,000 of the distance from the equator to the north pole through Paris. Now, it is defined by the ] as the distance travelled by ] in absolute ] in 1/299,792,458 of a ]. This is approximately the distance from floor to hip bone on the average barefoot man.

The symbol for metre is '''m'''. Decimal multiples and submultiples of the metre, such as '']'' (1000 metres) and '']'' (1/100 metre), are indicated by adding ]es to ''metre'' (see ] below).

== History ==
The word '']'' is from the Greek ''metron'' ({{polytonic|μέτρον}}), "a ]" via the French ''mètre''. Its first recorded usage in English meaning this unit of length 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 ] with a half-] of one ]. The other suggested defining the metre as one ten-millionth of the length of the Earth's ] along a quadrant, that is the distance from the equator to the north pole. In 1791, the ] selected the meridional definition over the pendular definition because the force of ] 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 ] commissioned an expedition led by ] and ], lasting from 1792 to 1799, which measured the length of the ] between ] and ]. This portion of the meridian, which also passes through ], was to serve as the basis for the length of the quarter meridian, connecting the ] with the ]. However, in 1793, France adopted the metre based on provisional results from the expedition as its official unit of length. Although it was later determined that 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 ] 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 ] (Convention du Mètre) of 1875 mandated the establishment of a permanent ] (BIPM: Bureau International des Poids et Mesures) to be located in ], France. This new organisation would preserve the new prototype metre and ] when constructed, distribute national metric prototypes, and would maintain comparisons between them and non-metric measurement standards. This organisation created a new prototype bar in 1889 at the first ] (CGPM: Conférence Générale des Poids et Mesures), establishing the ''International Prototype Metre'' as the distance between two lines on a standard bar of an alloy of ninety percent ] and ten percent ], measured at the melting point of ice.

In 1893, the standard metre was first measured with an ] by ], the inventor of the device and an advocate of using some particular ] of ] as a standard of distance. By 1925, ] was in regular use at the BIPM. However, the International Prototype Metre remained the standard until 1960, when the eleventh ] defined the metre in the new ] system as equal to 1,650,763.73 ]s of the ]-] ] in the ] of the ]-86 ] in a ]. The original international prototype of the metre is still kept at the BIPM under the conditions specified in 1889.

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 ] and the ]:

:''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.''<ref></ref>

Note that this definition had the effect of fixing 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. An intended byproduct of the 17<sup>th</sup> CGPM’s definition was that it enabled scientists to measure their lasers’ wavelengths with one-fifth the uncertainty. To further facilitate reproducibility from lab to lab, the 17<sup>th</sup> CGPM also made the iodine-stabilised Helium-Neon laser “a recommended radiation” for realising the metre.<ref>Reference: by the NIST.</ref> Today’s best determination of the wavelength of this laser is λ<sub>HeNe</sub>&nbsp;=&nbsp;632.991<FONT SIZE="-2">&nbsp;</FONT>398<FONT SIZE="-2">&nbsp;</FONT>22&nbsp;nm with an estimated relative standard uncertainty ''(U)'' of ± &nbsp;2.5&nbsp;×&nbsp;10<sup>-11</sup>. This uncertainty is currently the limiting factor in laboratory realisations of the metre as it is several orders of magnitude poorer than that of the second (''U''&nbsp;=&nbsp;1&nbsp;×&nbsp;10<sup>-14</sup>). Consequently, a practical realisation of the metre is usually delineated (not defined) today in labs as 1,579,800.298<FONT SIZE="-2">&nbsp;</FONT>728 ±&nbsp;0.000<FONT SIZE="-2">&nbsp;</FONT>039 wavelengths of Helium-Neon laser light in a vacuum.

===Timeline of definition===
* ]] &mdash; The ] decides that the length of the new metre would be equal to the length of a ] with a half-] of one ].

* ]] &mdash; The French National Assembly accepts the proposal by the ] that the new definition for the metre be equal to one ten-millionth of the length of the Earth's ] along a quadrant through Paris, that is the distance from the equator to the north pole.

* 1795 &mdash; Provisional metre bar constructed of ].

* ]] &mdash; The French National Assembly specifies the platinum metre bar, which is constructed on ] ] and deposited in the ], as the final standard.

* ]] &mdash; The first ] (CGPM) defines the length as the distance between two lines on a standard bar of an alloy of ] with ten percent ], measured at the melting point of ice.

* ]] &mdash; The seventh CGPM adjusts the definition of the length to be the distance, at 0&nbsp;°], between the axes of the two central lines marked on the prototype bar of platinum-iridium, this bar being subject to one standard ] and supported on two cylinders of at least one centimetre diameter, symmetrically placed in the same horizontal plane at a distance of 571 millimetres from each other.

* ]] &mdash; The eleventh CGPM defines the length to be equal to 1,650,763.73 ]s in ] of the ] corresponding to the transition between the 2p<sup>10</sup> and 5d<sup>5</sup> quantum levels of the ]-86 ].

* ]] &mdash; The seventeenth CGPM defines the length as equal to the distance travelled by ] in ] during a time interval of 1/299 792 458 of a ].

==SI prefixed forms of metre==
]es are often employed to denote decimal multiples and submultiples of the metre. The most commonly used factors of metre are listed below in '''bold'''.<ref>The term “most commonly used” is based on those with more than 5 million Google hits on the American spelling.</ref>

{| border="1" cellpadding="4" cellspacing="0" style="margin: 1em 1em 1em 0; background: #f5f5f5; border: 2px #525252 solid; border-collapse: collapse; font-size: 95%;"
|-
|colspan="3" style="background:#454545" align="center"|<font color=WHITE>Submultiples</font>||colspan="1" style="background:#454545" align="center"|<font color=WHITE></font>||colspan="3" style="background:#454545" align="center"|<font color=WHITE>Multiples</font>
|-
|style="background:#d4d4d4" align="center"|'''Factor'''
|style="background:#d4d4d4" align="center"|'''Name'''
|style="background:#d4d4d4" align="center"|'''Symbol'''
|style="background:#d4d4d4" align="center"|
|style="background:#d4d4d4" align="center"|'''Factor'''
|style="background:#d4d4d4" align="center"|'''Name'''
|style="background:#d4d4d4" align="center"|'''Symbol'''
|-
|align="center"|10<sup>&minus;1</sup>
|decimetre
|align="center"|dm
|
|align="center"|10<sup>1</sup>
|decametre
|align="center"|dam
|-
|align="center"|10<sup>&minus;2</sup>
|'''centimetre'''
|align="center"|cm
|
|align="center"|10<sup>2</sup>
|hectometre
|align="center"|hm
|-
|align="center"|10<sup>&minus;3</sup>
|'''millimetre'''
|align="center"|mm
|
|align="center"|10<sup>3</sup>
|'''kilometre'''
|align="center"|km
|-
|align="center"|10<sup>&minus;6</sup>
|'''micrometre'''
|align="center"|µm
|
|align="center"|10<sup>6</sup>
|megametre
|align="center"|Mm
|-
|align="center"|10<sup>&minus;9</sup>
|'''nanometre'''
|align="center"|nm
|
|align="center"|10<sup>9</sup>
|gigametre
|align="center"|Gm
|-
|align="center"|10<sup>&minus;12</sup>
|picometre
|align="center"|pm
|
|align="center"|10<sup>12</sup>
|terametre
|align="center"|Tm
|-
|align="center"|10<sup>&minus;15</sup>
|femtometre (fermi)
|align="center"|fm
|
|align="center"|10<sup>15</sup>
|petametre
|align="center"|Pm
|-
|align="center"|10<sup>&minus;18</sup>
|attometre
|align="center"|am
|
|align="center"|10<sup>18</sup>
|exametre
|align="center"|Em
|-
|align="center"|10<sup>&minus;21</sup>
|zeptometre
|align="center"|zm
|
|align="center"|10<sup>21</sup>
|zettametre
|align="center"|Zm
|-
|align="center"|10<sup>&minus;24</sup>
|yoctometre
|align="center"|ym
|
|align="center"|10<sup>24</sup>
|yottametre
|align="center"| Ym
|}

==Equivalents in other units==
{| class="wikitable"
|-
! SI value !! Other unit
|-
| 1 metre || 10000/254 ≈ 39.37 ]es
|-
| 2.54 centimetres || 1 inch
|-
| 1 nanometre || 10 ]s
|}

==See also==
*]
*] (International System of Units)
*]
*] for comparisons with other units
*]
*]

== References ==
* Bureau International des Poids et Mesures. . URL accessed on ] ].
* ———. . URL accessed on ] ].
* Penzes, William B. at the U.S. National Institute of Standards and Technology Precision Engineering Division (] ]). . URL accessed on ] ].
* U.S. National Institute of Standards and Technology (October 2000). :
** . URL accessed on ] ].
** . URL accessed on ] ].
** . URL accessed on ] ].

== Notes ==
<references />

== External links ==
* at U.S. NIST <!-- Information there not yet included in article -->
* By Tibo Qorl (Translated by Sibille Rouzaud)

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Revision as of 14:25, 15 November 2006

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