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 the fact that there were 9,192,631,770 transitions between two hyperfine ground-state energy levels every second. This number defined the second, as opposed to the old definition of there being 86,400 seconds in one day. A good atomic clock was accurate to one part in 1,014, and therefore would take about 3 million years to lose or gain a second.
Four atomic clocks are used in each of the many satellites of the Global Positioning System and comparisons of electromagnetic-wave travel times enable positions on Earth to be measured very precisely. The clocks are also used by geophysicists to monitor variations in the spin rate of Earth, and the drifting of the continents.
