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The caesium standard is a primary frequency standard in which electronic transitions between the two hyperfine ground states of caesium-133 atoms are used to control the output frequency. The first caesium clock was built by Louis Essen
Louis Essen
in 1955 at the National Physical Laboratory in the UK.[1] and promoted worldwide by Gernot M. R. Winkler of the USNO. Caesium
Caesium
atomic clocks are 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
International System of Units
(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. See also[edit]

Atomic clock Rubidium standard

References[edit]

^ L. Essen, J.V.L. Parry (1955). "An Atomic Standard of Frequency and Time Interval: A Caesium
Caesium
Resonator". Nature. 176 (4476): 280. Bibcode:1955Natur.176..280E. doi:10.1038/176280a0.  ^ Markowitz, W.; Hall, R.; Essen, L.; Parry, J. (1958). "Frequency of Cesium in Terms of Ephemeris Time". Physical Review Letters. 1 (3): 105. Bibcode:1958PhRvL...1..105M. doi:10.1103/PhysRevLett.1.105. 

 This article incorporates public domain material from the General Services Administration
General Services Administration
document "Federal Standard 1037C" (in support of MIL-STD-188).

External links[edit]

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Caesium
clocks.

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