A caesium standard is a highly constant frequency standard whose frequency
is derived from the electronic transition between two hyperfine levels of the caesium-133 atom. The change of state between the two levels occurs at 9,192,631,770 hertz (Hz), 9.192... GHz. This frequency could be measured in the 1960s and the second defined in the system of units
(SI) was derived from it. Caesium standards belong to the measurement standards and have an extremely high frequency stability of 10exp-11 to 10exp-12. The frequency drift is 10exp-13. The high frequency constancy is guaranteed if no external influences by the earth's magnetic field or electromagnetic interference fields affect the caesium standard.