The
metre or
meter[See American and British English spelling differences#-re, -er] is the basic unit of length in the
International System of Units (SI). 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 one ten-millionth of the distance from the Equator to the North Pole through Paris. In 1983, the metre was redefined as the distance travelled by
light in
free space in of a
second.
[The BIPM does not distinguish between quantum vacuum and free space. (CGPM, 1984), retrieved from BIPM database (BIPM, n.d.) on 24 August 2008. ]The symbol for metre is
m. Decimal multiples such as
kilometre and
centimetre are indicated by adding
SI prefixes to
metre.
History
The word
metre is from the Greek (
métron), "a
measure", via the French
mètre. It was first introduced in modern usage (
metro cattolico) by Italian scientist
Tito Livio Burattini in his work
Misura Universale in 1675, in order to rename the
universal measure unit proposed by
John Wilkins in 1668. Its first recorded usage in English meaning this unit of length is from 1797.
Meridional definition
In the eighteenth century, there were two favoured approaches to the definition of the standard unit of length. One approach suggested defining the metre as the length of a
pendulum with a half-
period of one
second, a '
seconds pendulum'. The other approach 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 half meridian, connecting the North Pole with the Equator.
However, in 1793, France adopted as its official unit of length a 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 because of miscalculation of the flattening of the Earth, this length became the standard. The circumference of the
Earth through the poles is therefore slightly more than forty million metres.
Prototype metre bar
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 standards when constructed, distribute national metric prototypes, and maintain comparisons between them and non-metric measurement standards. The 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 composed of an alloy of ninety percent
platinum and ten percent
iridium, measured at the melting point of ice.
The original international prototype of the metre is still kept at the BIPM under the conditions specified in 1889.
A discussion of measurements of a standard metre bar and the errors encountered in making the measurements is found in a NIST document.
[ ]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.
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:
This definition effectively fixed 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 realizations of the metre are still
delineated by counting the required number of wavelengths of light along the distance. Three major factors limit the accuracy attainable with laser interferometers:
- Uncertainty in vacuum wavelength of the source,
- Uncertainty in the refractive index of the medium,
- Laser count resolution of the interferometer.
Use of the interferometer to define the metre is based upon the relation:
where λ is the determined wavelength;
c is the speed of light in ideal vacuum;
n is the
refractive index of the medium in which the measurement is made; and
f is the frequency of the source. In this way the length is related to one of the most accurate measurements available: frequency.
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. For purposes of delineating the metre, the BIPM currently considers the HeNe laser wavelength to be as follows: with an estimated relative standard uncertainty (U) of . 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 (). Consequently, a practical realisation of the metre is usually delineated (not defined) today in labs as wavelengths of helium-neon laser light in a vacuum.Timeline of definition
- 1791 March 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.
- 1799 December 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.
- 1927 October 6 — The seventh CGPM adjusts the definition of the metre 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.
- 1960 October 20 — The eleventh CGPM defines the metre 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.
- 1983 October 21 — The seventeenth CGPM defines the metre as equal to the distance travelled by light in vacuum during a time interval of of a second.
SI prefixed forms of metre
SI prefixes are often employed to denote decimal multiples and submultiples of the metre, as shown in the table below.Spelling
Two spellings of the name of the unit are common in English: metre is preferred among the majority of countries in the English-speaking world except in the United States, where the spelling is meter.
The most recent official brochure, written in 2006, about the International System of Units (SI), Bureau international des poids et mesures, was written in French by the International Bureau of Weights and Measures. An English translation (using the spelling: metre) is included to make the SI standard "more widely accessible".
In 2008, the U.S. English translation published by the U.S. National Institute of Standards and Technology chose to use meter in accordance with the United States Government Printing Office Style Manual.
The spelling (-)meter for measuring devices, is the only correct spelling regardless of country, such as: parking meter, speedometer. (Meter, the device, has the same derivation as the metre detailed in this article.
The spelling (-)meter is used for the poetic words monometer, dimeter, trimeter, tetrameter, pentameter, hexameter, heptameter, and octameter.Equivalents in other units
Within this table, "inch" means "international inch".
"≈" means "is approximately equal to".
"≡" means "equals by definition" or equivalently, "is exactly equal to".See also
- ISO 1 – standard reference temperature for length measurements
