NATIONAL PHYSICAL LABORATORY (C) CROWN COPYRIGHT / SCIENCE PHOTO LIBRARY NATIONAL PHYSICAL LABORATORY (C) CROWN COPYRIGHT / SCIENCE PHOTO LIBRARY
Caesium atomic clock. In this clock, atoms of vapourised caesium-133 oscillate between two energy levels as they pass back and forth between magnets at each end of the resonator (long cylinder, lower left). Counting these oscillations is the basis of the standard second, where one second is about 9193 million oscillations. The development of the first caesium atomic clock by Louis Essen and Jack Parry in 1955 led to the replacement of the astronomical second with the atomic second as the standard unit of time. Photographed at the National Physical Laboratory, Teddington, UK.
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