:: The evolution of the metre's definitions ::

1793: The metre was defined as the ten-millonth part of a quarter of a terrestrian meridian. The definition of the unit of weight was linked to it, since it is the weight of a cubic decimetre of water. These two definitions are the "natural and universal" basis of the metric system.

1799: These definitions were theoretically kept, but the law added: "The platinum metre and kilogramme deposited in the National Archives are the final standards." These material standards, representatives of theoretical definitions, became actually the practical and legal basis of the metric system.

1872: It was decided producing prototype metres by using the original metre preserved in the French Archives as a reference. The original metre and kilogramme, called Legal Metre and Legal Kilogramme are the 1799 ones.

1875: The Metre Convention was signed.

1878-1889: Thirty prototype metres and fourty prototype kilogrammes were created and measured. It required comparing - with an accuracy never reached - the new line standards with the new x-section ones, and comparing them with the Legal Metre too, which is an end standard. It involved developping a special measuring apparatus and a definite and reproducible temperature scale.
A metre prototype and a kilogramme prototype were chosen, so that they could become the International Prototypes. The national prototypes were distributed. The International Prototypes were deposited in the IBWM on 28 September 1889 where they are still kept nowadays.

1887: Michelson suggested using optical interferometres to measure lengths. Afterwards, he received the Nobel prize in physics in 1907, mainly for his work in metrology.

1892-1893: Michelson and Benoît used the Michelson's interferometre in the IBWM to determine the metre value in wavelengths of the red line of cadmium.

1906: The above-mentionned measurement was confirmed by Benoît, Fabry and Pérot by means of Pérot and Fabry's interferometre.

1921-1936: The national prototypes were checked for the first time by comparing them with each other and with the International Prototype. There were new improved determinations of the prototypes coefficient of expansion.

1927: There was the International Agreement that defines the angstrom, based on the wavelength of the red line of cadmium determined in 1893 and 1906. The angstrom as so defined will be used as a unit of length in spectroscopy and in atomic physics until it is abandonned in 1960.

1952: The ICWM decided to go into the possibility of redefining the metre according to a light's wavelength and created for that purpose the Advisory Committe on the Metre Definition (that is nowadays the Advisory Committe on Lengths).

1960: The GCWM adopted a definition of the metre based on the wavelength in vacuum of the radiation corresponding to the transition between specified levels of the krypton 86 atom.
In the IBWM, measuring the line measures according to this wavelength replaced comparing line measures. New facilities were installed to do these measures by optical interferometry.

1975: The GCWM recommended for the speed of light in vacuum a value that resulted from the measurement of the wavelength and the frequency of one laser.

1983: The GCWM redefined the meter as the length of the distance covered by the speed of light in a vacuum during a precise fraction of second. It invited the ICWM to establish instructions for this new definition implementation. The ICWM, because it had anticipated this invitation, indicated general methods to link directly lengths to the metre as defined. There is among these methods the use of the wavelength of one of the recommended five radiations of lasers or spectroscopic lamps. The wavelengths and frequencies values, as well as their associated uncertainties, of these radiations are specified in the metre's definition implementation.
In the IBWM, the comparison of laser frequencies by optical beatings completes the measurement of line standards according to the wavelengths of these lasers.
1987: The IBWM began a new series of international comparisons of wavelengths of laser radiations - by optical interferometry - and of frequencies - by beating, in order to check the exactness of the practical realizations based on the new definition. The latter included comparisons of laser components, especially absorption cells that contain the atoms or molecules to which laser is a slave, as well as overall comparisons (optics, discharge tube, absorption and electronic cell).

1992: The ICWM decided to reduce uncertainties because of the works done in the national laboratories and in the IBWM. These uncertainties are the ones on the recommended radiations emitted by some lasers and that appear in the implementation. The ICWM decided to increase the number of these radiations from five to eight.

1997: The ICWM modified the 1992 implementation, by adding four new laser radiations, which increased their number to twelve, and by reducing again the uncertainties associated to the radiations of some lasers.
Works were going on in the IBWM and elsewhere to identify the factors that limit the reproducibility of lasers, wavelength and frequency standards.

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