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Resolution 1 of the CGPM (1967/68)
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of the CGPM at its 13th meeting (1967/68)

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SI unit of time (second)*   (2)

The 13th Conférence Générale des Poids et Mesures (CGPM),


  • that the definition of the second adopted by the Comité International des Poids et Mesures (CIPM) in 1956 (Resolution 1) and ratified by Resolution 9 of the 11th CGPM (1960), later upheld by Resolution 5 of the 12th CGPM (1964), is inadequate for the present needs of metrology,
  • that at its meeting of 1964 the Comité International des Poids et Mesures (CIPM), empowered by Resolution 5 of the 12th CGPM (1964), recommended, in order to fulfil these requirements, a caesium atomic frequency standard for temporary use,
  • that this frequency standard has now been sufficiently tested and found sufficiently accurate to provide a definition of the second fulfilling present requirements,
  • that the time has now come to replace the definition now in force of the unit of time of the Système International d'Unités by an atomic definition based on that standard,
  • decides

    1. The SI unit of time is the second defined as follows:
      "The second is the duration of 9 192 631 770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium 133 atom";
    2. Resolution 1 adopted by the CIPM at its meeting of 1956 and Resolution 9 of the 11th CGPM are now abrogated.

    Comptes Rendus de la 13e CGPM (1967/68), 1969, 103
    Metrologia, 1968, 4(1), 41-45


    At its 1997 meeting, the CIPM affirmed that this definition refers to a caesium atom at rest at a thermodynamic temperature of 0 K.

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