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Practical realizations of the definitions of some important units
Recommended values of standard frequencies
SI Brochure: Appendix 2

The following values are recommended by the CIPM for applications including the practical realization of the metre (MeP) and secondary representations of the second (SRS):


[Page last updated: 11 July 2013]
Note: All the radiations in the list (apart from the 87Rb 6.8 GHz transition) may be used for the practical realization of the metre (MeP).
Those radiations marked [SRS] may also be used as secondary representations of the second.

Graphical summary:

Background:

    The list of recommended radiations was first published by the CIPM in 1983 (see CI-1983, Recommendation 1) in the mise en pratique of the definition of the metre. This specified that the metre should be realized by one of the following methods:

    1. by means of the length l of the path travelled in vacuum by a plane electromagnetic wave in a time t; this length is obtained from the measured time t, using the relation l = c0 · t and the value of the speed of light in vacuum c0 = 299 792 458 m s–1,
    2. by means of the wavelength in vacuum lambda of a plane electromagnetic wave of frequency f; this wavelength is obtained from the measured frequency f using the relation lambda = c0/f and the value of the speed of light in vacuum c0 = 299 792 458 m s–1,
    3. by means of one of the radiations from the list given here, whose stated wavelength in vacuum or whose stated frequency can be used with the uncertainty shown, provided that the given specifications and accepted good practice are followed.
    For the full list of specifications, the reader is referred to the original Recommendation and to the various updates that have since been approved by the CIPM. Most of these updates have been recommended by the CCL, based on the work of the CCL Working Group on the Mise en Pratique (MePWG).

    In 2005 the MePWG and the CCL-CCTF Joint Working Group on Secondary Representations of the Second merged into a single CCL-CCTF Frequency Standards Working Group, which was charged with producing and maintaining a single list of Recommended values of standard frequencies for applications including the practical realization of the metre and secondary representations of the second. The drop-down menu at the top of this page gives access to the list being developed by this group. Where full data sheets are not yet available, links are given to relevant decisions of the CIPM or CC.

Appendix 2 of the SI Brochure

The mises en pratique are prepared by the relevant Consultative Committees, and, after approval by the CIPM, are then published in electronic form here on the BIPM website, where they may be revised more frequently than if they were printed in the SI Brochure.

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