ATOMIC CLOCK

a type of clock that uses certain resonance frequencies of atoms to keep time with extreme accuracy. The electronic components of atomic clocks are regulated by the frequency of the microwave electromagnetic radiation emitted or absorbed by the quantum transition (energy change) of an atom or molecule. In an atomic clock these quantum transitions produce extremely regular waves of electromagnetic radiation; like the recurrent events in other types of clocks, these waves are then counted. A beam of cesium atoms emerges from an oven and passes through an inhomogeneous magnet A, which The cesium-beam atomic clock is the most accurate standard of atomic time (q.v.) and frequency. In 1967 the 13th General Conference on Weights and Measures redefined the SI unit of time, the second, in terms of the cesium standard so as to equal the second of Ephemeris Time (q.v.). The Conference defined the second as the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the cesium-133 atom.