– the intergovernmental organization through which Member States act together
     on matters related to measurement science and measurement standards.
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CIPM, 1964: Declaration
Version française
Atomic and molecular frequency standards

The Comité International des Poids et Mesures,

empowered by Resolution 5 of the 12th Conférence Générale des Poids et Mesures to name atomic or molecular frequency standards for temporary use for time measurements in physics,

declares that the standard to be employed is the transition between the hyperfine levels F = 4, M = 0 and F = 3, M = 0 of the ground state 2S1/2 of the caesium 133 atom, unperturbed by external fields, and that the frequency of this transition is assigned the value 9 192 631 770 hertz.

Procès-Verbaux des Séances du CIPM (1964), p. 26
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