G-M

The metre (or meter, see spelling differences) 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. more...

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Historically, the metre 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 on the average barefoot man.

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 prefixes to metre (see table below).

History

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.

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, 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 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 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 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 Metre 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.

Read more at Wikipedia.org