![]() ![]() While the second is the only base unit to be explicitly defined in terms of the caesium standard, the majority of SI units have definitions that mention either the second, or other units defined using the second. 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. 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. ![]() 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, Δ ν Cs, of exactly 9 192 631 770 Hz. Winkler of the United States Naval Observatory.Ĭaesium atomic clocks are one of the most accurate time and frequency standards, and serve as the primary standard for the definition of the second in the International System of Units (SI) (the modern form of the metric system). The first caesium clock was built by Louis Essen in 1955 at the National Physical Laboratory in the UK. The caesium standard is a primary frequency standard in which the photon absorption by transitions between the two hyperfine ground states of caesium-133 atoms is used to control the output frequency. Primary frequency standard A caesium atomic fountain used as part of an atomic clock ![]()
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