Caesium standard
A caesium standard or caesium atomic clock is a primary frequency standard in which electronic transitionsbetween the two hyperfine ground states of caesium-133 atoms are used to control the output frequency. The first caesium clock was built by Louis Essenin 1955 at the National Physical Laboratory in the UK.[1]
Caesium clocks are the most accurate commercially produced time and frequency standards, and serve as the primary standard for the definition of the second in SI (the metric system). By definition, radiation produced by the transition between the two hyperfine ground states of caesium (in the absence of external influences such as the Earth’s magnetic field) has a frequency of exactly 9,192,631,770 Hz. That value was chosen so that the caesium second equalled, to the limit of human measuring ability in 1960 when it was adopted, the existing standard ephemeris second based on the Earth‘s orbit around the Sun.[2] Because no other measurement involving time had been as precise, the effect of the change was less than the experimental uncertainty of all existing measurements.
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http://en.m.wikipedia.org/wiki/Caesium_standard
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