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[1] 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[2] 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 metrologists were authorized to travel there to duplicate the marks on to their own bars for regional and national prototypes.[3]
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.[4]