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The metre, or '''meter''', is a measure of in absolute Vacuum during a time interval of 1/299,792,458 of a Second . The symbol of the metre is '''m'''. Historically, the metre was intended to be, and is very nearly, the ten-millionth part of the distance from the equator to the north pole. Multiples and subdivisions of the metre, such as ''kilometre'' (1000 metres) and ''centimetre'' (1/100 metres), are indicated by adding SI Prefix es to ''metre''. HISTORY The word ''metre'' is from the Greek ''metron'' (), "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 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 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 metre, more accurate measurements of this meridian than 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 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. 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 the melting point of ice. 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. 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 exactly fixes the speed of light in a vacuum at 299,792,458 metres per second. Definitions based on the physical properties of light are more precise and reproducible because the properties of light are considered to be universally constant. Timeline of definition
SI PREFIXES To denote multiples and subdivisions of the metre, SI Prefixes may be applied. The most commonly used are listed in bold. EQUIVALENTS IN OTHER UNITS SEE ALSO
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