Resolution 5 of the 21st CGPM (1999)

Revision of the mise en pratique of the definition of the metre

The 21st Conférence Générale des Poids et Mesures,

recalling that

  • in 1983 the 17th General Conference adopted a new definition of the metre,
  • in the same year the General Conference invited the International Committee
  • to draw up instructions for the practical realization of the metre (the mise en pratique,
  • to choose radiations which can be recommended as standards of wavelength for the interferometric measurement of length and draw up instructions for their use,
  • to pursue studies to improve these standards and, in due course, to extend or revise these instructions,
    • in response to this invitation the International Committee made recommendations in 1983 and again in 1992 concerning the practical realization of the metre,

    considering that

    • science and technology continue to demand improved accuracy in the realization of the metre,
    • since 1992 work in national laboratories, the Bureau International des Poids et Mesures and elsewhere has substantially improved the reproducibility of radiations which are suitable for the practical realization of the metre,
    • such work has also substantially reduced the uncertainty in the determined values of the frequencies and vacuum wavelengths of some of these radiations,
    • a revision of the list of recommended radiations is desirable for many applications, which include not only the direct realization of the metre by means of optical interferometry for practical length measurement, but also spectroscopy, atomic and molecular physics and the determination of fundamental constants,
    • the accuracy already achieved and advances now foreseeable in certain space-time measurements require the practical realization of the definition of the metre to be considered in the context of the theory of general relativity,

    welcomes the adoption by the International Committee in 1997 of a revised mise en pratique of the definition of the metre,

    and recommends that national laboratories pursue experimental and theoretical research on optical wavelength and frequency standards, including the development of new techniques for the comparison of different standards over a wide range of wavelength and frequency to improve yet further the experimental basis of the International System of Units.

    DOI : 10.59161/CGPM1999RES5E

    The reader should note that the official version of this Resolution is the French text

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