– the intergovernmental organization through which Member States act together
     on matters related to measurement science and measurement standards.
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The former Prototype Metre

Michelson's interferometer, 1892/3

Michelson's interferometer, 1892/3
Length measurement was a core function at the BIPM since its foundation. Indeed, the 1889 International Prototype Metre was a copy of the 1799 Mètre des Archives that was itself established during the famous measurements of the Earth's meridian between Dunkerque and Barcelona.

In many ways it is amazing that an apparently simple piece of metal remained adequate for an international measurement standard until 1960. This was despite the development of spectroscopic light sources, our understanding of atomic structures, and the advance of interferometry as a technique for distance measurement. The reason is, perhaps, associated with two things: first, the excellence of optical microscopes and comparators which were widely used in dimensional metrology; secondly, suitable narrow linewidth light sources were low power and their coherence length – the distance over which precise distance measurement could be made by interferometry – was still very much less than a metre. Only in 1960 was the metal bar replaced by a definition of the metre based on a certain number of wavelengths of light. This was after many years of research to establish the metre, initially in terms of the cadmium red line, by such giants of optics as Michelson, who worked at the BIPM where he constructed one of his famous interferometers.

Subsequently some of the best national laboratory researchers worked for many years to identify krypton as the best option for an "optical metre". The definition of the metre based on a certain number of wavelengths of light emitted from a specified transition in a single isotope of krypton under controlled conditions lasted until 1983. The krypton lamp was a difficult device to operate and required careful and painstaking operation in order to perform at its best: a remarkable achievement for classical metrology. By a curious coincidence the krypton definition was adopted at just around the time that optical technologies were revolutionized by the development of the laser. The metrological potential of the laser was immediately recognized and substantial resources were devoted to early studies of frequency stabilization techniques.