Atomic Clocks

    At one end of the caesium clock is an oven which evaporates atoms of caesium from the surface of a piece of the metal. These atoms will have their electrons in one of the two arrangements described above. A magnet is used to separate them and discard those with the higher energy.

    The clock's quartz crystal oscillator is tuned as accurately as possible to 9192631770 Hz. It controls a source of radio waves aimed at the atoms with the lower energy. If the crystal's frequency is correct, many of the atoms have their states changed. Caesium I operating at the NPL. The quartz oscillator was contained in the collection of electronics against the wall and radio waves travelled to the beam tube in the foreground through the overhead pipe. Most of the apparatus surrounding the beam tube is for pumping air out of it. The light coloured coils around the outside cancel out the Earth's magnetic field.

    At the other end of the beam tube is another magnet which separates those atoms which have been changed from those which have not. The atoms in the changed state are counted by a detector. If the number which have been changed starts to fall, it is because the frequency of the quartz crystal has drifted. In that case, an automatic control system adjusts the crystal oscillator until the number of atoms being changed reaches a maximum again. Preventing the crystal's frequency from changing keeps the clock accurate. An electronic counter converts the oscillation frequency to pulses at intervals of exactly one second.

    The principle was first suggested by Dr. Isador Rabi of Columbia University in the 1930s.

    Other types of atomic clock use either hydrogen or rubidium. The caesium atomic clock was used to redefine the second because it was the first type of atomic clock to work.

    In 1948, the National Bureau of Standards (NBS) in the USA built the world's first atomic clock. It used molecules of ammonia instead of caesium atoms and the beam tube was a copper pipe wrapped around the clock face. It was never used for timekeeping because it was less reliable than quartz clocks, with an error of about one second in four months.

    Once Caesium I was working, Louis Essen and Jack Parry began to improve their design. Caesium II, built in 1959, had a longer beam tube so the atoms were exposed to the radio waves for longer. This made it more accurate. It kept time to within one second in 2000 years.

    The first commercially available caesium clock, the Atomichron, went on sale in October 1956. By the time the HP5060A clock was produced in 1964, technology had improved the accuracy of atomic clocks to one second in 6000 years.