# Metre

The metre, or meter, is a measure of length, approximately equal to 3.28 feet. As the basic unit of length in the metric system and in the International System of Units (SI: Système International d'Unités), the metre is defined as equal to the length of the path travelled by light 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 prefixes to metre.

## History

The word metre 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 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.

File:Platinum-Iridium meter bar.jpg
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 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.

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.

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.<ref>Resolution 1 of the 17th CGPM (1983): Definition of the metre</ref>

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

• 1791March 30 — The French National Assembly accepts the proposal by the French Academy of Sciences that the new definition for the metre be equal to 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.
• 1795 — Provisional metre bar constructed of brass.
• 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 centimetre diameter, symmetrically placed in the same horizontal plane at a distance of 571 millimetres from each other.
• 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 prefixes

To denote multiples and subdivisions of the metre, SI prefixes may be applied. The most commonly used are listed in bold.

Multiple Name Symbol Multiple Name Symbol
100 metre m
10−1 decimetre dm 101 decametre dam
10−2 centimetre cm 102 hectometre hm
10−3 millimetre mm 103 kilometre km
10−6 micrometre µm 106 megametre Mm
10−9 nanometre nm 109 gigametre Gm
10−12 picometre pm 1012 terametre Tm
10−15 femtometre 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

## Equivalents in other units

SI value Other unit
1 metre 10000/254 ≈ 39.37 inches
2.54 centimetres 1 inch
1 nanometre 10 ångströms