# metre

1 metre =
SI units
1000 mmcm
US customary / Imperial units
ftin
The metre or meter[1](symbol: m) is the fundamental unit of length in the International System of Units (SI). The metre was originally defined by a prototype object meant to represent 110 000 000 the distance between the poles and the Equator. Today, it is defined as 1299 792 458 of a light-second.

Because it is the base unit of length in the SI, all SI units which involve length (such as area or speed) are defined relative to the metre. Additionally, due to the metre being the only SI base unit used to measure a vector (e.g. displacement), all vector units are defined relative to the metre. However, decimal multiples and submultiples of the metre— such as kilometre (1000 metres) and centimetre (0.01 metres)— can be formed by adding SI prefixes to metre (see the table below).

## Etymology

The word is from the Greek metron (μέτρον), "a " 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

Historical International Prototype Metre bar, made of an alloy of platinum and iridium, which 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 maintain comparisons between them and non-metric measurement standards. This organization 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 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 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.

### 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.[2]

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. Today's best determination of the wavelength of the relevant transition in 127I2 used for this purpose is λ = 632 991 212.58 fm with an estimated relative standard uncertainty (U) of 2.1 × 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)[3] Consequently, a practical realisation of the metre is usually delineated (not defined) today in labs as 1 579 800.762 042(33) 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 metre 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 metre 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 metre bar constructed of brass.
• 1799December 10 — The French National Assembly specifies the platinum metre 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 centimetre diameter, symmetrically placed in the same horizontal plane at a distance of 571 millimetres 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 2p10 and 5d5 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 metre

Orders of
magnitude (length)

in E notation
1 E-24 m
1 E-23 m
1 E-22 m
1 E-21 m
1 E-20 m
1 E-19 m
1 E-18 m
1 E-17 m
1 E-16 m
1 E-15 m
1 E-14 m
1 E-13 m
1 E-12 m
1 E-11 m
1 E-10 m
1 E-9 m
1 E-8 m
1 E-7 m
1 E-6 m
1 E-5 m
1 E-4 m
1 E-3 m
1 E-2 m
1 E-1 m
1 E0 m
1 E+1 m
1 E+2 m
1 E+3 m
1 E+4 m
1 E+5 m
1 E+6 m
1 E+7 m
1 E+8 m
1 E+9 m
1 E+10 m
1 E+11 m
1 E+12 m
1 E+13 m
1 E+14 m
1 E+15 m
1 E+16 m
1 E+17 m
1 E+18 m
1 E+19 m
1 E+20 m
1 E+21 m
1 E+22 m
1 E+23 m
1 E+24 m
1 E+25 m
1 E+26 m
/

SI prefixes are often employed to denote decimal multiples and submultiples of the metre, as shown in the table below.
Multiple Name Symbol Multiple Name Symbol
100metrem
101decametredam10–1decimetredm
102hectometrehm10–2centimetrecm
103kilometrekm10–3millimetremm
106megametreMm10–6micrometrem
109gigametreGm10–9nanometrenm
1012terametreTm10–12picometrepm
1015petametrePm10–15femtometrefm
1018exametreEm10–18attometream
1021zettametreZm10–21zeptometrezm
1024yottametreYm10–24yoctometreym

## Equivalents in other units

Metric unit
expressed in non-SI unit
Non-SI unit
expressed in metric unit
1 metre10−4mil              1 mil104metres
1 metre39.37inches              1 inch0.0254metres
1 centimetre0.3937inch 1 inch2.54centimetres
1 millimetre0.03937inch 1 inch25.4millimetres
1 metre1×1010Ångström 1 Ångström1×10-10metre
1 nanometre10Ångström 1 Ångström100picometres

## References

A Meter or metre is a unit of measurement.

Meter or metre may also refer to:
• Meter (device), a measuring instrument
• Meter (electronics), an instrument for measuring quantities in electronic circuits

International System of Units (abbreviated SI from the French Le Système international d'unités) is the modern form of the metric system.
1 millimetre =
SI units
010−3 m 0 cm
US customary / Imperial units
010−3 ft 010−3 in
The millimetre (American spelling: millimeter, symbol mm
1 centimetre =
SI units
010−3 m 0 mm
US customary / Imperial units
010−3 ft 0 in
A centimetre (American spelling: centimeter, symbol cm
U.S. customary units, also known in the United States as English units[1] (but see English unit) or standard units, are units of measurement that are currently used in the USA, in some cases alongside units from SI (the International System of Units
Imperial units or the Imperial system is a collection of units, first defined in the British Weights and Measures Act of 1824, later refined (until 1959) and reduced.
1 foot =
SI units
0 m 0 mm
US customary / Imperial units
0 yd 0 in
A foot (plural: feet or foot;[1] symbol or abbreviation: ft or, sometimes,
1 inch =
SI units
010−3 m 0 mm
US customary / Imperial units
010−3 ft 010−3 yd

An inch (plural: inches; symbol or abbreviation: in or, sometimes,
American English (AmE, AE, AmEng, USEng, en-US), also known as United States English or U.S. English, is a set of dialects of the English language used mostly in the United States.
The international system (SI) of units defines seven SI base units: physical units defined by an operational definition.

All other physical units can be derived from these base units: these are known as SI derived units. Derivation is by dimensional analysis.
Length is the long dimension of any object. The length of a thing is the distance between its ends, its linear extent as measured from end to end. This may be distinguished from height, which is vertical extent, and width or breadth
International System of Units (abbreviated SI from the French Le Système international d'unités) is the modern form of the metric system.
This article or section is in need of attention from an expert on the subject.
equator is an imaginary line on the Earth's surface equidistant from the North Pole and South Pole. It thus divides the Earth into a Northern Hemisphere and a Southern Hemisphere. The equators of other planets and astronomical bodies are defined analogously.
speed of light in a vacuum is an important physical constant denoted by the letter c for constant or the Latin word celeritas meaning "swiftness".[1] It is the speed of all electromagnetic radiation, including visible light, in a vacuum.
Area is a physical quantity expressing the size of a part of a surface. The term Surface area is the summation of the areas of the exposed sides of an object.

### Units

Units for measuring surface area include:
square metre = SI derived unit

Speed is the rate of motion, or equivalently the rate of change in position, many times expressed as distance d traveled per unit of time t.

Speed is a scalar quantity with dimensions distance/time; the equivalent vector quantity to speed is known as
1 kilometre =
SI units
0 m 0106 mm
US customary / Imperial units
0 ft 0 mi
A kilometre (American spelling: kilometer, symbol km
1 centimetre =
SI units
010−3 m 0 mm
US customary / Imperial units
010−3 ft 0 in
A centimetre (American spelling: centimeter, symbol cm
An SI prefix (also known as a metric prefix) is a name or associated symbol that precedes a unit of measure (or its symbol) to form a decimal multiple or submultiple.
v and A).]] A pendulum is an object that is attached to a pivot point so it can swing freely. This object is subject to a restoring force that will accelerate it toward an equilibrium position.
Periodicity is the quality of occurring at regular intervals or periods (in time or space) and can occur in different contexts:
• A clock marks time at periodic intervals.
• A metronome ticks at periodic intervals of time.

second (SI symbol: s), sometimes abbreviated sec., is the name of a unit of time, and is the International System of Units (SI) base unit of time.

SI prefixes are frequently combined with the word second to denote subdivisions of the second, e.g.
meridian (or line of longitude) is an imaginary arc on the Earth's surface from the North Pole to the South Pole that connects all locations with a given longitude. The position of a point on the meridan is given by the latitude.
French Academy of Sciences (French: Académie des sciences) is a learned society, founded in 1666 by Louis XIV at the suggestion of Jean-Baptiste Colbert, to encourage and protect the spirit of French scientific research.
The Bureau des Longitudes is a French scientific institution, founded by decree of June 25 1795 and charged with the improvement of nautical navigation, standardisation of time-keeping, geodesy and astronomical observation.
Jean Baptiste Joseph, chevalier Delambre (September 19, 1749 Amiens - August 19, 1822 Paris) was a French mathematician and astronomer.

After a childhood fever, he suffered from very sensitive eyes, and believed that he would soon go blind.
Pierre François André Méchain (August 16, 1744 – September 20, 1804) was a French astronomer and surveyor who, with Charles Messier, was a major contributor to the early study of deep sky objects and comets.
meridian (or line of longitude) is an imaginary arc on the Earth's surface from the North Pole to the South Pole that connects all locations with a given longitude. The position of a point on the meridan is given by the latitude.
Commune of
Dunkerque

Location

Coordinates