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\\\'\\\'This article is about the unit of length. For other uses of \\\'\\\'meter\\\'\\\' or \\\'\\\'meter\\\'\\\', see \\\'\\\'meter (disambiguation)\\\'\\\'.\\\'\\\'

Template:Unit of length The \\\'\\\'\\\'meter\\\'\\\'\\\', or \\\'\\\'\\\'metre\\\'\\\'\\\' (UK), is a measure of length. It is the basic unit of length in the metric system and in the International System of Units (SI), used around the world for general and scientific purposes. Historically, the meter was defined by the French Academy of Sciences as 1/10,000,000 of the distance from the equator to the north pole through Paris. Now, it is defined by the International Bureau of Weights and Measures as the distance travelled by light in absolute vacuum in 1/299,792,458 of a second. This is approximately the distance from floor to hip bone on the average barefoot man.

The symbol for meter is \\\'\\\'\\\'m\\\'\\\'\\\'. Decimal multiples and submultiples of the meter, such as \\\'\\\'kilometer\\\'\\\' (1000 meters) and \\\'\\\'centimeter\\\'\\\' (1/100 meter), are indicated by adding SI prefixes to \\\'\\\'meter\\\'\\\' (see table below).

History

The word \\\'\\\'meter\\\'\\\' is from the Greek \\\'\\\'metron\\\'\\\' (Template:Polytonic), \\\"a measure\\\" 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 meter as the length of a pendulum with a half-period of one second. The other suggested defining the meter 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 meter, 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 quarter meridian, connecting the North Pole with the Equator. However, in 1793, France adopted the meter based on provisional results from the expedition as its official unit of length. Although it was later determined that the first prototype meter bar was short by a fifth of a millimeter 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 meters.

In the 1870s and in light of modern precision, a series of international conferences were held to devise new metric standards. The meter 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 meter and kilogram 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 General Conference on Weights and Measures (CGPM: Conférence Générale des Poids et Mesures), establishing the \\\'\\\'International Prototype meter\\\'\\\' as the distance between two lines on a standard bar of an alloy of ninety percent platinum and ten percent iridium, measured at the melting point of ice.

In 1893, the standard meter 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 meter remained the standard until 1960, when the eleventh CGPM defined the meter 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 meter 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 meter with its current definition, thus fixing the length of the meter in terms of time and the speed of light:

\\\'\\\'The meter is the length of the path travelled by light in vacuum during a time interval of 1/299 792 458 of a second.\\\'\\\'

Note that this definition had the effect of fixing the speed of light in a vacuum at precisely 299,792,458 meters per second. Although the meter is now \\\'\\\'defined\\\'\\\' in terms of time-of-flight, actual laboratory realisations of the meter are still \\\'\\\'delineated\\\'\\\' by counting the required number of wavelengths of light along the distance. An intended byproduct of the 17 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 CGPM also made the iodine-stabilised Helium-Neon laser “a recommended radiation” for realising the meter. Today’s best determination of the wavelength of this laser is λ_HeNe = 632.991 398 22 nm with an estimated relative standard uncertainty \\\'\\\'(U)\\\'\\\' of ±  2.5 × 10. This uncertainty is currently the limiting factor in laboratory realisations of the meter as it is several orders of magnitude poorer than that of the second (\\\'\\\'U\\\'\\\' = 1 × 10). Consequently, a practical realisation of the meter 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.

Timeline of definition

• 1790May 8 — The French National Assembly decides that the length of the new meter would be equal to the length of a pendulum with a half-period of one second.

• 1791March 30 — The French National Assembly accepts the proposal by the French Academy of Sciences that the new definition for the meter be equal to one ten-millionth of the length of the Earth\\\'s meridian along a quadrant through Paris, that is the distance from the equator to the north pole.

• 1795 — Provisional meter bar constructed of brass.

• 1799December 10 — The French National Assembly specifies that the platinum meter bar, constructed on 23 June 1799 and deposited in the National Archives, as the final standard.

• 1889September 28 — The first General Conference on Weights and Measures (CGPM) defines the length as the distance between two lines on a standard bar of an alloy of platinum with ten percent iridium, measured at the melting point of ice.

• 1927October 6 — The seventh CGPM adjusts the definition of the length to be the distance, at 0 °C, between the axes of the two central lines marked on the prototype bar of platinum-iridium, this bar being subject to one standard atmosphere of pressure and supported on two cylinders of at least one centimeter diameter, symmetrically placed in the same horizontal plane at a distance of 571 millimeters from each other.

• 1960October 20 — The eleventh CGPM defines the length to be equal to 1,650,763.73 wavelengths in vacuum of the radiation corresponding to the transition between the 2p and 5d quantum levels of the krypton-86 atom.

• 1983October 21 — The seventeenth CGPM defines the length as equal to the distance travelled by light in vacuum during a time interval of 1/299 792 458 of a second.

SI prefixed forms of meter

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

Equivalents in other units

See also

• Metric system
• SI (International System of Units)
• SI prefix
• Conversion of units for comparisons with other units
• Orders of magnitude (length)
• Speed of light

References

• Bureau International des Poids et Mesures. The BIPM and the evolution of the definition of the meter. URL accessed on 2006 June 3.
• ———. Resolutions of the CGPM. URL accessed on 2006 June 3.
• Penzes, William B. at the U.S. National Institute of Standards and Technology Precision Engineering Division (2005 December 29). Time Line for the Definition of the Meter. URL accessed on 2006 June 3.
• U.S. National Institute of Standards and Technology (October 2000). The NIST Reference on Constants, Units, and Uncertainty: International System of Units (SI):
  • SI base units. URL accessed on 2006 June 3.
  • Definitions of the SI base units. URL accessed on 2006 June 3.
  • Historical context of the SI: meter. URL accessed on 2006 June 3.

Notes

1. Resolution 1 of the seventeenth CGPM (1983): Definition of the meter
2. Reference: \\\'\\\'Time Line for the Definition of the Meter\\\'\\\' by the NIST.
3. The term “most commonly used” is based on those with more than 5 million Google hits on the American spelling.

External links

• Length—Evolution from Measurement Standard to a Fundamental Constant at U.S. NIST
• The History of the Meter By Tibo Qorl (Translated by Sibille Rouzaud)

• SI base units
• Units of length