Metre

metermmetresmetersnmcmnanometersmetricalattometerskilometers
The metre (Commonwealth spelling and BIPM spelling ) or meter (American spelling ) (from the French unit mètre, from the Greek noun μέτρον, "measure") is the base unit of length in the International System of Units (SI).wikipedia
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American and British English spelling differences

spelling differencesorsee spelling differences
The metre (Commonwealth spelling and BIPM spelling ) or meter (American spelling ) (from the French unit mètre, from the Greek noun μέτρον, "measure") is the base unit of length in the International System of Units (SI).
One outcome is the British distinction of meter for a measuring instrument from metre for the unit of length.

Base unit (measurement)

base unitbase unitsfundamental quantity
The metre (Commonwealth spelling and BIPM spelling ) or meter (American spelling ) (from the French unit mètre, from the Greek noun μέτρον, "measure") is the base unit of length in the International System of Units (SI).
In the International System of Units, there are seven base units: kilogram, metre, candela, second, ampere, kelvin, and mole.

Length

widthlengthsbreadth
The metre (Commonwealth spelling and BIPM spelling ) or meter (American spelling ) (from the French unit mètre, from the Greek noun μέτρον, "measure") is the base unit of length in the International System of Units (SI).
In the International System of Units (SI), the basic unit of length is the metre and is now defined in terms of the speed of light.

Seconds pendulum

a pendulum that has a period of 2 secondsone second pendulum
In 1671 Jean Picard measured the length of a "seconds pendulum" (a pendulum with a period of two seconds) at the Paris observatory. As the figure of the Earth could be inferred from variations of the seconds pendulum length with latitude, the United States Coast Survey instructed Charles Sanders Peirce in the spring of 1875 to proceed to Europe for the purpose of making pendulum experiments to chief initial stations for operations of this sort, in order to bring the determinations of the forces of gravity in America into communication with those of other parts of the world; and also for the purpose of making a careful study of the methods of pursuing these researches in the different countries of Europe.
In 1790, one year before the metre was ultimately based on a quadrant of the Earth, Talleyrand proposed that the metre be the length of the seconds pendulum at a latitude of 45°.

Unit of measurement

unitunits of measurementweights and measures
The SI unit symbol is m. Another geodesist with metrology skills was to play a pivotal role in the process of internationalization of weights and measures, Carlos Ibáñez e Ibáñez de Ibero who would become the first president of both the International Geodetic Association and the International Committee for Weights and Measures.
The metre is a unit of length that represents a definite predetermined length.

Unit of length

lengthunits of lengthDistance
According to the decision of the Congress of the United States, the British Parlementary Standard from 1758 was introduced as the unit of length.
The base unit in the International System of Units (SI) is the metre, defined as "the length of the path travelled by light in vacuum during a time interval of 1⁄299792458 seconds."

Carlos Ibáñez e Ibáñez de Ibero

Another geodesist with metrology skills was to play a pivotal role in the process of internationalization of weights and measures, Carlos Ibáñez e Ibáñez de Ibero who would become the first president of both the International Geodetic Association and the International Committee for Weights and Measures. A member of the Preparatory Committee since 1870 and Spanish representative at the Paris Conference in 1875, Carlos Ibáñez e Ibáñez de Ibero intervened with the French Academy of Sciences to rally France to the project to create an International Bureau of Weights and Measures equipped with the scientific means necessary to redefine the units of the metric system according to the progress of sciences.
His activities resulted in the distribution of a platinum and iridium prototype of the metre to all States parties to the Metre Convention during the first meeting of the General Conference on Weights and Measures in 1889.

Metric system

metricmetric unitsmetric unit
A member of the Preparatory Committee since 1870 and Spanish representative at the Paris Conference in 1875, Carlos Ibáñez e Ibáñez de Ibero intervened with the French Academy of Sciences to rally France to the project to create an International Bureau of Weights and Measures equipped with the scientific means necessary to redefine the units of the metric system according to the progress of sciences.
In its modern form, it consists of a set of base units: metre for length, kilogram for mass, second for time, ampere for electrical current, kelvin for temperature, candela for luminous intensity and mole for quantity.

Metrology

metrologicalmetrologistlegal metrology
Another geodesist with metrology skills was to play a pivotal role in the process of internationalization of weights and measures, Carlos Ibáñez e Ibáñez de Ibero who would become the first president of both the International Geodetic Association and the International Committee for Weights and Measures.
In March 1791, the metre was defined.

Earth's circumference

circumference of the Earthcircumference of EarthEquatorial circumferences
The metre was originally defined in 1793 as one ten-millionth of the distance from the equator to the North Pole along a great circle, so the Earth's circumference is approximately 40,000 km.
Both the metre and the nautical mile were originally defined as a subdivision of the Earth's circumference; today the circumference around the poles is very nearly 40,000 km and 360 × 60 nautical miles long.

Paris meridian

Meridian of ParisArago medallionFrench meridian arc
This portion of the Paris meridian, was to serve as the basis for the length of the half meridian connecting the North Pole with the Equator.
As the metre had to be equal to one ten-million of this distance, it was defined as 0,513074 toises or 443,2936 lignes of the Toise of Peru (see below).

History of the metre

Mètre des Archivesinternational prototype metreinternational prototype meter
"The unit of length to which all distances measured in the Coast Survey are referred is the French metre, an authentic copy of which is preserved in the archives of the Coast Survey Office. It is the property of the American Philosophical Society, to whom it was presented by Mr. Hassler, who had received it from Tralles, a member of the French Committee charged with the construction of the standard metre by comparison with the toise, which had served as unit of length in the measurement of the meridional arcs in France and Peru. It possesses all the authenticity of any original metre extant, bearing not only the stamp of the Committee but also the original mark by which it was distiguished from the other bars during the operation of standarding. It is always designated as the Committee metre" (French : Mètre des Archives).
As the metre had to be equal to one ten-millionth of this distance, it was defined as 0.513074 toise or 3 feet and 11.296 lines of the Toise of Peru.

Charles Sanders Peirce

PeirceC. S. PeirceCharles S. Peirce
As the figure of the Earth could be inferred from variations of the seconds pendulum length with latitude, the United States Coast Survey instructed Charles Sanders Peirce in the spring of 1875 to proceed to Europe for the purpose of making pendulum experiments to chief initial stations for operations of this sort, in order to bring the determinations of the forces of gravity in America into communication with those of other parts of the world; and also for the purpose of making a careful study of the methods of pursuing these researches in the different countries of Europe.
Also in 1877, he proposed measuring the meter as so many wavelengths of light of a certain frequency, the kind of definition employed from 1960 to 1983.

French Geodesic Mission

Spanish-French Geodesic MissionGeodesic Mission to Peruscientific mission
From 1801 to 1812 France adopted this definition of the metre as its official unit of length based on results from this expedition combined with those of the Geodesic Mission to Peru.
When an International Commission for Weights and Measures was convened in Paris to settle the true length of the metre, it adopted on 22 June 1799 a standard metre based on the length of the half meridian connecting the North pole with the Equator.

Pendulum

pendulumssimple pendulumpendula
In 1671 Jean Picard measured the length of a "seconds pendulum" (a pendulum with a period of two seconds) at the Paris observatory.
In the discussions leading up to the French adoption of the metric system in 1791, the leading candidate for the definition of the new unit of length, the metre, was the seconds pendulum at 45° North latitude.

Speed of light

clight speedspeed of light in vacuum
To further reduce uncertainty, the 17th CGPM in 1983 replaced the definition of the metre with its current definition, thus fixing the length of the metre in terms of the second and the speed of light:
It is exact because by international agreement a metre is defined as the length of the path travelled by light in vacuum during a time interval of 1⁄299792458 second.

Dunkirk

DunkerqueMalo-les-BainsDunkirk, France
The French Academy of Sciences commissioned an expedition led by Jean Baptiste Joseph Delambre and Pierre Méchain, lasting from 1792 to 1799, which attempted to accurately measure the distance between a belfry in Dunkerque and Montjuïc castle in Barcelona at the longitude of Paris Panthéon.
Using this measurement and the latitudes of the two cities they could calculate the distance between the North Pole and the Equator in classical French units of length and hence produce the first prototype metre which was defined as being one ten millionth of that distance.

General Conference on Weights and Measures

CGPMConférence Générale des Poids et MesuresCGPM conference
The organisation distributed such bars 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 90% platinum and 10% iridium, measured at the melting point of ice.
Initially the Metre Convention was only concerned with the kilogram and the metre, but in 1921 the scope of the treaty was extended to accommodate all physical measurements and hence all aspects of the metric system.

Electromagnetic spectrum

spectrumspectraspectral
However, the International Prototype Metre remained the standard until 1960, when the eleventh CGPM defined the metre in the new International System of Units (SI) as equal to 1650763.73 wavelengths of the orange-red emission line in the electromagnetic spectrum of the krypton-86 atom in a vacuum.

Wavelength

wavelengthswave lengthsubwavelength
However, the International Prototype Metre remained the standard until 1960, when the eleventh CGPM defined the metre in the new International System of Units (SI) as equal to 1650763.73 wavelengths of the orange-red emission line in the electromagnetic spectrum of the krypton-86 atom in a vacuum. 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 length.
The wavelengths of sound frequencies audible to the human ear (20 Hz–20 kHz) are thus between approximately 17 m and 17 mm, respectively.

Atomic clock

atomic clocksatomiccaesium clock
Nowadays the practical realisation of the metre is possible everywhere thanks to the atomic clocks embedded in GPS satellites.
The definitions of other physical units, e.g., the volt and the metre, rely on the definition of the second.

Foot (unit)

feetftfoot
: 1 metre is nearly equivalent to 3feet 3 3⁄8inches.
Since the International Yard and Pound Agreement of 1959, one foot is defined as 0.3048 meter exactly.

Atmospheric pressure

barometric pressureair pressurepressure
Pressure measures force per unit area, with SI units of Pascals (1 pascal = 1 newton per square metre, 1N/m 2 ).

Angstrom

Åångströmangstroms
The angstrom or ångström is a unit of length equal to m; that is, one ten-billionth of a metre, 0.1 nanometre, or 100 picometres.

Yard

yardsydTotal return yards
Since 1959 it is by international agreement standardized as exactly 0.9144 meters.