Declaration of the 53rd CIPM (1964)

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 9 182 631 770 hertz.

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

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