Difference between revisions of "Metre"
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===Relativity=== | ===Relativity=== | ||
| + | The metre is considered to be a unit of [[proper length]]. As such, the definition is only adequate in itself for measurements in an area where the local gravitational field can be considered to be uniform, a condition which is met by almost all measurements on the surface of the Earth (the Earth's gravitational field varies by about one part in 10<sup>16</sup> per vertical metre, which is far less than the [[measurement uncertainty]] in Earth-based length measurements).<ref name="1-CI-2002">Recommendation 1 (CI-2002), [http://www.bipm.org/utils/en/pdf/CIPM2002-EN.pdf 91st Meeting of the International Committee on Weights and Measures], 2002.</ref> For astronomical measurements, the effects of [[general relativity]] cannot be ignored and the definition of the metre must be completed with a description of the [[metric tensor (general relativity)|metric tensor]] which defines the gravitational field.<ref>{{citation | last = Guinot | first = B. | title = Application of general relativity to metrology | journal = Metrologia | year = 1997 | volume = 34 | issue = 3 | pages = 261–90 | doi = 10.1088/0026-1394/34/3/9}}.</ref> | ||
==Realization== | ==Realization== | ||
| − | The metre can be realized either directly, by measuring the time taken for light to travel a certain distance, or by [[interferometry]].<ref | + | The metre can be realized either directly, by measuring the time taken for light to travel a certain distance, or by [[interferometry]].<ref name="1-CI-2002"/><ref name="MeP">{{citation | title = ''Mise en pratique'' for the definition of the metre | url = http://www.bipm.org/en/si/si_brochure/appendix2/mep.html | publisher = Interntional Bureau for Weights and Measures | date = 2009 | accessdate = 2010-08-13}}.</ref> |
===Time-of-flight measurements=== | ===Time-of-flight measurements=== | ||
Revision as of 07:41, 13 August 2010
The metre (symbol: m), spelled meter in the United States, is the SI unit of length and one of the seven base units in the International System of Units. The name is derived from the Greek μέτρον (metron) meaning "measure", and gives rise to the term "metric system", a system of units of which the metre is one.
The metre was conceived in the aftermath of the French Revolution (1789) as a replacement for the old units of length that were associated with the ancien régime. Although there was initially considerable resistance to the adoption of the new units in France (including an official reversion to the mesures usuelles ["normal units"] for a period), the metre gained following in continental Europe during the mid nineteenth century, particularly in scientific usage, and was consacrated as an international measurement unit by the Metre Convention of 1875.
Definition
The current definition of the metre was agreed at the 17th General Conference on Weights and Measures (CGPM) in 1983:[1]
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.
This definition has the effect that the speed of light in vacuum has an exact, defined value in SI units: 299 792 458 m s−1.
Relativity
The metre is considered to be a unit of proper length. As such, the definition is only adequate in itself for measurements in an area where the local gravitational field can be considered to be uniform, a condition which is met by almost all measurements on the surface of the Earth (the Earth's gravitational field varies by about one part in 1016 per vertical metre, which is far less than the measurement uncertainty in Earth-based length measurements).[2] For astronomical measurements, the effects of general relativity cannot be ignored and the definition of the metre must be completed with a description of the metric tensor which defines the gravitational field.[3]
Realization
The metre can be realized either directly, by measuring the time taken for light to travel a certain distance, or by interferometry.[2][4]
Time-of-flight measurements
Interferometry
| Source | Stabilizing line (127I2) | Frequency THz |
Wavelength nm |
ur |
|---|---|---|---|---|
| Helium–neon laser | a16 component, R(127) 11-5 transition | 473.612 353 604(10) | 632.991 212 579(13) | 2.1 × 10−11 |
| Nd:YAG laser | a10 component, R(56) 32-0 transition | 563.260 223 513(5) | 532.245 036 104(5) | 8.9 × 10−12 |
For an unstabilized helium–neon laser (Ne 3s2→2p4 transition):[5]
- f = 473.612 7 THz
- λ = 632.990 8 nm
- ur = 1.5 × 10−6
History
Notes and references
Notes
References
- ↑ The International System of Units (SI), 8th ed.; International Bureau of Weights and Measures: Sèvres, France, 2006; p 112. ISBN 92-822-2213-6, <http://www.bipm.org/utils/common/pdf/si_brochure_8_en.pdf>.
- ↑ 2.0 2.1 Recommendation 1 (CI-2002), 91st Meeting of the International Committee on Weights and Measures, 2002.
- ↑ Guinot, B. Application of general relativity to metrology. Metrologia 1997, 34 (3), 261–90. DOI: 10.1088/0026-1394/34/3/9.
- ↑ Mise en pratique for the definition of the metre; Interntional Bureau for Weights and Measures, 2009, <http://www.bipm.org/en/si/si_brochure/appendix2/mep.html>. (accessed 13 August 2010).
- ↑ Recommendation 2 (CI-2007), 96th Meeting of the International Committee on Weights and Measures, 2007.
External links
 |
See also the corresponding article on Wikipedia. |
- meter (metre) at Sizes.com
| Error creating thumbnail: Unable to save thumbnail to destination |
This page is currently licensed under the Creative Commons Attribution 3.0 Unported license and any later versions of that license. |
