periods of the radiation corresponding to the transition between two designated energy levels (hyperfine levels of the ground state) of caesium-133. [P1.1]
is: the SI unit of time, one of the seven base units. [P1.1]
is defined: as the duration of periods of the radiation corresponding to the transition between two designated energy levels (hyperfine levels of the ground state) of caesium-133. [P1.1]
See caesium atomic clock.
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