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The metre (American English: '''meter''') 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 metre was defined by the French Academy Of Sciences as the length between two marks on a platinum-iridium bar, which was designed to represent 1/10,000,000 of the distance from the equator to the north pole through Paris. Today, 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 . The symbol for metre is m. Decimal multiples and submultiples of the metre, such as '' Kilometre '' (1000 metres) and '' Centimetre '' (1/100 metre), are indicated by adding SI Prefix es to ''metre'' (see Table below). ETYMOLOGY The word ''" via the French ''mètre''. Its first recorded usage in English meaning this unit of length is from 1797. HISTORY 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. Prototype metre bar In the 1870s and in light of modern precision, a series of international conferences were held to devise new metric standards. The and ten percent Iridium , measured at 0 degrees Celsius 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 Wavelength s 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. 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. 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.Reference: ''Time Line for the Definition of the Meter'' by the NIST. 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) NIST-F1 Cesium Fountain Atomic Clock .. 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. Timeline of definition
SI PREFIXED FORMS OF METRE SI Prefix 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. EQUIVALENTS IN OTHER UNITS SEE ALSO
REFERENCES
NOTES EXTERNAL LINKS
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