Metre
This article is about the unit of length. For other uses of metre
or meter
, see meter (disambiguation).), "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.
 |
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.[Resolution 1 of the seventeenth CGPM (1983): Definition of the metre]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
*
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 that 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 water.
*
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 2p
10 and 5d
5 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.
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 |
| SI value | Other unit |
|---|
| 1 metre | 10000/254 ≈ 39.37 inches |
| 2.54 centimetres | 1 inch |
| 1 nanometre | 10 ångströms |
*
Metric system*
SI (International System of Units)
*
SI prefix*
Conversion of units for comparisons with other units
*
Orders of magnitude (length)*
Speed of light*
Lengthâ€"Evolution from Measurement Standard to a Fundamental Constant at U.S. NIST
*
The History of the Meter By Tibo Qorl (Translated by Sibille Rouzaud)
* Bureau International des Poids et Mesures.
The BIPM and the evolution of the definition of the metre. URL accessed on
2006 June 3.
* â€"â€"â€".
Resolutions of the CGPM. URL accessed on
2006 June 3.
* Penzes, William B. at the U.S. National Institute of Standards and Technology Precision Engineering Division (
2005 December 29).
Time Line for the Definition of the Meter. URL accessed on
2006 June 3.
* U.S. National Institute of Standards and Technology (October 2000).
The NIST Reference on Constants, Units, and Uncertainty: International System of Units (SI):
**
SI base units. URL accessed on
2006 June 3.
**
Definitions of the SI base units. URL accessed on
2006 June 3.
**
Historical context of the SI: meter. URL accessed on
2006 June 3.
