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Definitions for the purist
  1. P McCrory

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    WHAT IS A SECOND?

    Since 1967, the international standard for a second has been defined as the time it takes for 9 192 631 770 oscillations of the microwave radiation corresponding to the transition between two hyperfine levels of the ground state of an atom of celsium-133. It sounds complex and an extremely accurate method for measuring time although a more recent proposal using a ytterbium standard is superior by a factor of more than a hundred times.

    WHAT IS A METRE?

    The origins …

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