International Prototype Meter

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The meter 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 of that length was constructed in 1799 and housed at Pavillon de Breteuil near Paris. This is the headquarters for the International Bureau of Weights and Measures. Its French language acronym is BIPM.

It was discovered that this bar was a fraction of a millimeter too long. In 1889 the meter was redefined 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 is kept at the BIPM.

Selected metrologists were authorized to travel there to duplicate the marks on to their own bars for regional prototypes.^[1] The new bar served as standard until 1960 when the meter was redefined in terms of the wavelength of light emitted by the krypton-86 isotope. The meter was redefined yet again in 1989 in terms of the speed of light. The present speed of light is defined as 299,792,458 meters per second (in a vacuum) and is used to indirectly calculate the length of the meter.^[2]

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