International Prototype Meter
Encyclopedia
The metre
was originally defined as one ten-millionth of the distance between the North
Pole and the equator
at the longitude of Paris
. Because of the difficulty of reproducing this measurement, a platinum bar nominally of that length was constructed in 1799 and housed in the Archives de la République in Paris.
Because of the difficulty of precisely measuring the end-to-end length of a metal bar such as the 1799 prototype, the metre was redefined in 1889 as the distance between precision marks on a new 'X' shaped 90% platinum
10% iridium
bar at 0 °C. This alloy was used because it does not oxidize, is hard, can be highly polished, and expands or contracts very little with temperature changes. The bar was kept at the headquarters of the International Bureau of Weights and Measures
(BIPM) in Pavillon de Breteuil
near Paris. Selected metrologist
s were authorized to travel there to duplicate the marks on to their own bars for regional and national prototypes.
The new bar served as standard until 1960 when the metre was redefined in terms of the wavelength
of light emitted by the krypton-86
isotope. The metre was redefined yet again in 1983 in terms of the speed of light
. The speed of light is defined as 299,792,458 metres per second (in a vacuum) and is used to indirectly calculate the length of the metre.
Metre
The metre , symbol m, is the base unit of length in the International System of Units . Originally intended to be one ten-millionth of the distance from the Earth's equator to the North Pole , its definition has been periodically refined to reflect growing knowledge of metrology...
was originally defined as one ten-millionth of the distance between the North
North
North is a noun, adjective, or adverb indicating direction or geography.North is one of the four cardinal directions or compass points. It is the opposite of south and is perpendicular to east and west.By convention, the top side of a map is north....
Pole and the equator
Equator
An equator is the intersection of a sphere's surface with the plane perpendicular to the sphere's axis of rotation and containing the sphere's center of mass....
at the longitude of Paris
Paris
Paris is the capital and largest city in France, situated on the river Seine, in northern France, at the heart of the Île-de-France region...
. Because of the difficulty of reproducing this measurement, a platinum bar nominally of that length was constructed in 1799 and housed in the Archives de la République in Paris.
Because of the difficulty of precisely measuring the end-to-end length of a metal bar such as the 1799 prototype, the metre was redefined in 1889 as the distance between precision marks on a new 'X' shaped 90% platinum
Platinum
Platinum is a chemical element with the chemical symbol Pt and an atomic number of 78. Its name is derived from the Spanish term platina del Pinto, which is literally translated into "little silver of the Pinto River." It is a dense, malleable, ductile, precious, gray-white transition metal...
10% iridium
Iridium
Iridium is the chemical element with atomic number 77, and is represented by the symbol Ir. A very hard, brittle, silvery-white transition metal of the platinum family, iridium is the second-densest element and is the most corrosion-resistant metal, even at temperatures as high as 2000 °C...
bar at 0 °C. This alloy was used because it does not oxidize, is hard, can be highly polished, and expands or contracts very little with temperature changes. The bar was kept at the headquarters of the International Bureau of Weights and Measures
International Bureau of Weights and Measures
The International Bureau of Weights and Measures , is an international standards organisation, one of three such organisations established to maintain the International System of Units under the terms of the Metre Convention...
(BIPM) in Pavillon de Breteuil
Pavillon de Breteuil
Pavillon de Breteuil is a building located in Sèvres, France near Paris. It was inaugurated by Louis XIV in 1672. It is in the park of the former royal Château de Saint-Cloud, which was destroyed in 1870....
near Paris. Selected metrologist
Metrologist
Metrologists perform metrology work involving precision measurement and comparison of physical quantities such as mass, length, time, force, speed, voltage and current. They calibrate precision equipment which measures these physical units. They may also certify that outside standards of such...
s were authorized to travel there to duplicate the marks on to their own bars for regional and national prototypes.
The new bar served as standard until 1960 when the metre was redefined in terms of the wavelength
Wavelength
In physics, the wavelength of a sinusoidal wave is the spatial period of the wave—the distance over which the wave's shape repeats.It is usually determined by considering the distance between consecutive corresponding points of the same phase, such as crests, troughs, or zero crossings, and is a...
of light emitted by the krypton-86
Isotopes of krypton
There are 33 known isotopes of krypton from 69 to 101. Naturally occurring krypton is made of six stable isotopes, two of which may be slightly radioactive. Its spectral signature can be produced with some very sharp lines. 81Kr, the product of atmospheric reactions is produced with the other...
isotope. The metre was redefined yet again in 1983 in terms of the speed of light
Speed of light
The speed of light in vacuum, usually denoted by c, is a physical constant important in many areas of physics. Its value is 299,792,458 metres per second, a figure that is exact since the length of the metre is defined from this constant and the international standard for time...
. The speed of light is defined as 299,792,458 metres per second (in a vacuum) and is used to indirectly calculate the length of the metre.