Most types of clocks rely on the oscillation of a solid body, be it a pendulum, a balance-wheel, or a quartz crystal, but each suffers from the effects of temperature, pressure, and gravity. Time measuring devices have also depended on the spin of the Earth, but these suffer from seasonal effects and tidal friction. Atoms, however, vibrate a fixed number of times per second. Both the-U.S. National Bureau of Standards and the United Kingdom's National Physics Laboratory tried to take advantage of these vibrations.
In 1949 the Americans built a quartz clock that was synchronized by the 24-GHz vibrations of low-pressure gaseous ammonium molecules. The British, under the leadership of physicist Louis Essen (1908-1997), used the oscillations of an electrical circuit synchronized to the vibrations of caesium atoms, the first caesium clock being built in 1955. The caesium was kept in a tuneable microwave cavity and the clock relied on
more...