Atomic Clocks

In 1880, Pierre Curie and his brother Jacques discovered that crystals could be made to vibrate by applying an electric field to them, an effect known as piezoelectricity. By cutting crystals of quartz in different ways, they can be made to vibrate at almost any frequency. In 1927, Warren Marrison produced the first clock to work by counting the vibrations of a piece of quartz.

At the National Physical Laboratory, Louis Essen acheived greater precision by using rings of quartz instead of slabs. He found that variations in the frequency depended largely on the way in which the quartz was held in place. The Essen ring, developed in 1938, was a ring of quartz suspended by six silk threads. Clocks containing an Essen ring were accurate to one second in three years.

The Royal Greenwich Observatory bought its first quartz clock in 1939. In 1942, it started to take their official time from groups of quartz clocks at the Post Office Radio Branch in Dollis Hill, rather than from its own Shortt free pendulum clocks. In 1944, four banks of three quartz clocks, one of which is shown here, were installed in different cellars at the observatory and their times were compared electronically. The crystals in these clocks were often tuned to 100000 Hz (cycles per second).

The Greenwich Time Service began broadcasting radio signals at precise frequencies from its quartz clocks in 1944 but the BBC "pips", which began in 1924, were still produced by pendulum clocks until 1949. The speaking clock, which had been operating since 1936, switched to quartz technology in 1964. The change reduced its possible error from 0.1 seconds to 0.005 seconds and meant that it only had to be compared with the Greenwich clocks once every day instead of once every hour.

Improvements in electronic technology enabled quartz clocks to become smaller. By 1961, portable quartz clocks were being produced for navigation purposes and today, quartz crystals are found in most wristwatches.

How atomic clocks work

The part of an atomic clock which is responsible for keeping time is actually a quartz crystal oscillator. In most quartz clocks, the oscillator is tuned accurately when the clock is made but its frequency is never checked again. Over time, its frequency changes slightly but unpredictably, making the clock fast or slow.

The purpose of the complicated apparatus in an atomic clock is to check the frequency of the quartz oscillator continually, giving the clock its great accuracy.

An atom can be thought of as a collection of electrons orbiting a nucleus like planets around the Sun. Calculations using quantum mechanics show that only certain orbits are allowed. To move from a high orbit to a lower one, an electron must emit energy in the form of electromagnetic radiation (light or radio waves) of a particular frequency. This frequency depends on the energy difference between the two orbits. If an electron in the lower orbit is supplied with radiation of exactly the right frequency, it will jump to the higher orbit.

Each caesium atom contains 55 electrons. The last of these normally occupies an orbit which is much further from the nucleus than the rest. In this orbit, its energy can have two slightly different values, depending on a property called the "spin" of the electron. The energy difference between the two states corresponds to radio waves with a frequency of 9192631770 Hz (cycles per second). Atoms in these two states have slightly different magnetic properties.