Metre

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1 metre =
SI units
1000 mm 100 cm
US customary / Imperial units
3.281 ft 39.37 in

The metre or meter (symbol: m) is the fundamental unit of length in the International System of Units (SI). The metre was originally defined by a prototype object meant to represent 110 000 000 the distance between the poles and the Equator. Today, it is defined as 1299 792 458 of a light-second.

Because it is the base unit of length in the SI, all SI units which involve length (such as area or speed) are defined relative to the metre. Additionally, due to the metre being the only SI base unit used to measure a vector (e.g. displacement), all vector units are defined relative to the metre. However, decimal multiples and submultiples of the metre— such as kilometre (1000 metres) and centimetre (0.01 metres)— can be formed by adding SI prefixes to metre (see the table below).

Contents

  • 1 Etymology
  • 2 History
    • 2.1 Meridional definition
    • 2.2 Prototype metre bar
    • 2.3 Standard wavelength of krypton-86 emission
    • 2.4 Standard wavelength of helium-neon laser light
    • 2.5 Timeline of definition
  • 3 SI prefixed forms of metre
  • 4 Equivalents in other units
  • 5 See also
  • 6 References
  • 7 Notes
  • 8 External links

[edit] Etymology

The word metre is from the Greek metron (μέτρον), "a measure" via the French mètre. Its first recorded usage in English meaning this unit of length is from 1797.

[edit] History

[edit] Meridional definition

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 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, that is the distance from the equator to the north pole. In 1791, the French Academy of Sciences selected the meridional definition.

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.

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 Earth through the poles is approximately forty million metres.

[edit] Prototype metre bar

Historical International Prototype Metre bar, made of an alloy of platinum and iridium, that was the standard from 1889 to 1960.

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 when constructed, distribute national metric prototypes, and maintain comparisons between them and non-metric measurement standards. This organization 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 of an alloy of ninety percent platinum and ten percent iridium, measured at 0 degrees Celsius

[edit] 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. The original international prototype of the metre is still kept at the BIPM under the conditions specified in 1889.

[edit] 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:

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.[1]

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 17th CGPM’s definition was that it enabled scientists to measure the wavelength of their lasers with one-fifth the uncertainty. To further facilitate reproducibility from lab to lab, the 17th CGPM also made the iodine-stabilised Helium-Neon laser "a recommended radiation" for realising the metre.[2] Today's best determination of the wavelength of this laser is λHeNe = 632.991 398 22 nm with an estimated relative standard uncertainty (U) of ±  2.5 × 10-11. 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 = 5 × 10-16)[3]. Consequently, a practical realisation of the metre is usually delineated (not defined) today in labs as 1,579,800.298 728 ± 0.000 039 wavelengths of Helium-Neon laser light in a vacuum.

[edit] Timeline of definition

[edit] SI prefixed forms of metre

Orders of
magnitude (length)
in E notation

1 E-24 m
1 E-23 m
1 E-22 m
1 E-21 m
1 E-20 m
1 E-19 m
1 E-18 m
1 E-17 m
1 E-16 m
1 E-15 m
1 E-14 m
1 E-13 m
1 E-12 m
1 E-11 m
1 E-10 m
1 E-9 m
1 E-8 m
1 E-7 m
1 E-6 m
1 E-5 m
1 E-4 m
1 E-3 m
1 E-2 m
1 E-1 m

1 E0 m
1 E+1 m
1 E+2 m
1 E+3 m
1 E+4 m
1 E+5 m
1 E+6 m
1 E+7 m
1 E+8 m
1 E+9 m
1 E+10 m
1 E+11 m
1 E+12 m
1 E+13 m
1 E+14 m
1 E+15 m
1 E+16 m
1 E+17 m
1 E+18 m
1 E+19 m
1 E+20 m
1 E+21 m
1 E+22 m
1 E+23 m
1 E+24 m
1 E+25 m
1 E+26 m

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SI prefixes are often employed to denote decimal multiples and submultiples of the metre. The most commonly used factors of metre are listed below in bold.

Factor Name Symbol Factor Name Symbol
10−1 decimetre dm 101 decametre dam
10−2 centimetre cm 102 hectometre hm
10−3 millimetre mm 103 kilometre km
10−6 micrometre (micron) µm 106 megametre Mm
10−9 nanometre nm 109 gigametre Gm
10−12 picometre pm 1012 terametre Tm
10−15 femtometre (fermi) fm 1015 petametre Pm
10−18 attometre am 1018 exametre Em
10−21 zeptometre zm 1021 zettametre Zm
10−24 yoctometre ym 1024 yottametre Ym

[edit] Equivalents in other units

Metric unit
expressed in non-SI unit  
Non-SI unit
expressed in metric unit
1 metre 10−4 mil                1 mil 104 metres           
1 metre 39.37 inches                1 inch 0.0254 metres           
1 centimetre 0.3937 inch   1 inch 2.54 centimetres  
1 millimetre 0.03937 inch   1 inch 25.4 millimetres  
1 metre 1×1010 Ångström   1 Ångström 1×10-10 metre  
1 nanometre 10 Ångström   1 Ångström 100 picometres