Atomic Clocks

    He retired from his post of Deputy Chief Scientific Officer in 1972 and died in 1997, having won such awards as an OBE, the A.C. Popov Gold Medal from the Soviet Academy of Sciences and the Tompion Gold Medal of the Clockmakers' Company.

    The frequency of a pendulum is only constant if it swings freely. If it is interfered with, its speed will be changed. Unfortunately, if the pendulum is never pushed, it will eventually stop swinging and, without pushing something else, it will be unable to control a clock.

    In 1889, a clock was produced which had one pendulum swinging as freely as possible and another to control the clock mechanism. The design was gradually refined and in 1921, William Shortt and F Hope-Jones produced the Shortt-Synchronome free pendulum system shown here.

    The picture on the left shows a Shortt free pendulum in a display case. Normally, it would work in a special enclosure in a near vacuum. The free pendulum is connected electronically to a "slave" clock, a modified electrically wound pendulum clock. The picture on the right shows details of this clock.

    Every thirty seconds, the slave clock sends an electrical signal to the mechanism above the free pendulum, which gives it a tiny push to keep it swinging. As this mechanism resets itself, it sends a signal back to a synchronising unit on the slave clock, which adjusts the swinging of its pendulum to keep it exactly in step with the free one.

    Because there is no interference with the movement of the free pendulum except for a tiny push every thirty seconds, it keeps time very accurately - to within about one second per year. This is much more accurate than the slave clock on its own because the pendulum of this clock has to move part of the mechanism with every swing and needs a larger push to keep it moving than the free pendulum does.


    About a hundred clock systems like this were made for observatories all over the world. The first was installed at Edinburgh in 1921 and some were used at the Royal Greenwich Observatory from 1921 until 1942, when they were replaced by quartz crystal clocks.