NPL's contribution to the introduction of the caesium second

For centuries timekeeping was the preserve of astronomers. The fundamental unit of time measurement was the day, sub-divided by clocks for everyday use into hours, minutes and seconds. By the early 20th century, clocks were sufficiently stable to indicate that the length of the mean solar day varied, though their lack of intrinsic accuracy (the rate of a clock depended on its mechanical properties) ensured that the Earth's rotation remained the global reference standard for timekeeping. This situation changed fundamentally in June 1955, when Louis Essen and Jack Parry brought the first caesium atomic clock into operation at the National Physical Laboratory, in Teddington, UK. The atomic clock not only provided a much more stable timekeeper than the Earth's rotation; its reference was the frequency of an atomic transition - a fundamental constant of nature, determined by the laws of quantum mechanics. As a result all caesium clocks will run at essentially the same rate, limited only by noise processes and their local environment, regardless of time or place.