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The BIPM and the evolution of the definition of the metre

The CCL-CCTF merged list of recommended frequencies


The CCL and CCTF collaborate to update the list of frequencies recommended by the CIPM for applications including the practical realization of the metre (MeP) and secondary representations of the second (SRS).


A new comparison, CCL-K11, was initiated, designed to replace BIPM.L-K11; CCL-K11 is piloted by BEV supported by NMIJ, MIKES, NPL, and NRC as local nodes.

To date

The CCL and CCTF continue to monitor developments and regularly recommend to the CIPM updates of the list of frequencies recommended by the CIPM for applications including the practical realization of the metre (MeP) and secondary representations of the second (SRS).

Development of optical frequency combs


Around year 2000 a revolutionary new technique was demonstrated. Optical frequency combs were realized based on mode-locked femtosecond lasers, providing a phase-coherent connection between radiofrequency and optical frequencies and allowing direct frequency measurements in the optical range to be made in a single step.


The BIPM piloted the key comparison BIPM.L-K11, providing world-wide traceability for national realizations of the metre and a technical basis for the review of CMC claims.


The BIPM Length Section was closed and the ongoing key comparison BIPM.L-K11 stopped. Staff and resources were transferred to the BIPM Time Department.

Redefinition of the metre in terms of the second; stabilized lasers


The CGPM recommended a value for the speed of light in vacuum as a result of measurements of the wavelength and the frequency of laser radiation.


The CGPM redefined the metre as the length of the path travelled by light in vacuum during a specific fraction of a second. It invited the CIPM to draw up instructions for the practical realization of the new definition. The CIPM, having anticipated the above invitation, outlined general ways in which lengths can be directly related to the metre as newly defined. These included the wavelengths of five recommended laser radiations as well as those of spectral lamps. The wavelengths, frequencies and associated uncertainties were specified in the instructions for the practical realization of the definition.

At the BIPM, comparison of laser frequencies by beat-frequency techniques supplemented the measurement of linescales in terms of wavelengths of the same lasers.


In order to check the accuracy of practical realizations of the metre based upon the new definition, a new round of international comparisons of laser wavelengths by optical interferometry and frequency by beat-frequency techniques was begun at the BIPM. Such international comparisons comprised comparisons of individual components of the laser, in particular the absorption cells containing the atoms or molecules upon which the laser is stabilized, as well as comparisons of whole laser systems (optics, gas cells and electronics).


The CIPM decided, on the basis of new work in national laboratories and at the BIPM, to reduce significantly the uncertainties associated with the laser radiations recommended in 1983 and to increase their number from five to eight.


The CIPM modified the 1992 instructions for the practical realization of the definition by further reducing the uncertainties and increasing the number of recommended radiations from eight to twelve.

Work continued at the BIPM and elsewhere to identify the factors limiting the reproducibility of lasers as wavelength and frequency standards.

Redefinition of the metre in terms of wavelengths of light; the krypton 86 atom


International accord, using the above 1893 and 1906 determinations of the wavelength of the red line of cadmium, defining the ångström; the ångström thus defined was henceforth used as the spectroscopic unit of length until abandoned in 1960.


The CIPM decided to investigate the possibility of redefining the metre in terms of a wavelength of light, and established the Comité Consultatif pour la Définition du Mètre (now called the Consultative Committee for Length) for this purpose.


The CGPM adopted a definition of the metre in terms of the wavelength in vacuum of the radiation corresponding to a transition between specified energy levels of the krypton 86 atom.

At the BIPM, measurement of linescales in terms of this wavelength replaced comparisons of linescales between themselves; new equipment was installed for doing this by optical interferometry.

Optical interferometry and the first comparisons


Michelson proposed the use of optical interferometers for the measurement of length. He subsequently received the 1907 Nobel Prize for physics for, among other things, his metrological work.


The Michelson interferometer was used at the BIPM (by Michelson and Benoît) to determine the length of the metre in terms of the wavelength of the red line of cadmium.


The above measurement was confirmed by Benoît, Fabry and Perot using the interferometer made by Perot and Fabry.


Nobel Prize for physics for Ch.-Ed. Guillaume, then Director of BIPM, for his invention of invar.


First verification of the national prototypes by intercomparisons among themselves and by comparisons with the International Prototype. This included new and improved determinations of the thermal expansion of the metre bars.

Prototype metres


Decision taken to make prototype metres, with the original metre held in the Archives de France serving as the reference. (The original metre and kilogram, called the Mètre des Archives and Kilogramme des Archives, were constructed in 1799 to be one ten-millionth of a quadrant of the Earth and the mass of a cubic decimetre of water respectively.)


Convention of the Metre signed.


Renovation of the buildings of the Pavillon de Breteuil and construction of a laboratory; recruitment of staff; acquisition of specialized equipment for length and mass comparisons.


Preparation and measurement of thirty metre prototypes (and forty kilogram prototypes). This required matching, with an unprecedented precision, the new "X" cross-section metre line-standards to one another and to the 1799 metre (Mètre des Archives), which was an end standard. This entailed the development of some unique measuring equipment and of a reproducible, definable temperature scale. Selection of Metre and Kilogram prototypes which became the international prototypes. Distribution of the national prototypes. On 28 September 1889 the International Prototypes were deposited at the BIPM, where they remain today.

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The former International Prototype of the Metre