Sue Nelson reports

The "pips" have been used by millions of people in the UK and around the world to set their clocks and watches. Heard before the main news bulletins on radio, they have also preceded some of the great announcements of the 20th Century, from the invasion of Poland in 1939 to the death of Diana, Princess of Wales, in 1997.

Royal Observatory Greenwich: The home of time

To mark the special anniversary, the two original clocks that were used to time the pips have been returned to the Royal Observatory Greenwich, and will go on public display.

The timepieces have been in storage for several years, but, according to Jonathan Betts, the observatory's Curator of Horology, are still in perfect working order.

"All that's been required is some light cleaning and re-lubrication," he told BBC News Online. "They'll be ticking."

Electrical contacts

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The pips system was devised by the Ninth Astronomer Royal, Frank Dyson, in consultation with Frank Hope-Jones, the manufacturer of the free pendulum clock.

Time was marked by two weight-driven astronomical regulators - one was a back-up in case of breakage.

The clocks sent pulses up to Broadcasting house

Electrical contacts attached to the pendulum sent pulses from Greenwich to the BBC's headquarters at Broadcasting House where an oscillatory valve circuit converted the signal into the now famous six tones.

The clocks - No 2016 and No 2012 - were actually built in 1874 by E Dent and Company of London for use in finding objects in the night sky. Although still regarded as precision instruments in 1924, technological improvements meant that more accurate timekeepers were soon being used to regulate the Dent clocks.

The observatory's senior astronomer Robin Catchpole explains how the pips came about

They were eventually relieved of their pip duties in 1949.

"It was a wonderfully conceived system," said Jonathan Betts. "An audible signal, as astronomers have known for many years, is far more useful than any visual signal - you listen while you check your watch.

"Also, they were the first - everybody internationally has followed the BBC."

Atomic time

Today, the Corporation keeps its own time with two atomic clocks sited in the basement of Broadcasting House. These are kept in step by signals received from the Global Positioning Satellite (GPS) system and by information received from the Rugby radio transmitter facility which is operated by BT Aeronautical and Maritime under contract to the National Physical Laboratory (NPL).

The BBC now has its own atomic clocks

"It's a little known but interesting fact that the Greenwich Time Signal no longer gives Greenwich Mean Time," said the NPL's John Chambers. "Since 1972, all the time signals in the world have been based on atomic time."

This is far more regular than Greenwich Mean Time (GMT) which drifts as the speed of the Earth's rotation changes. Differences between the two mean that "leap seconds" occasionally need to be added to the time signal, by means of an extra pip. This keeps the two types of time roughly in step.

This oddity is one of the reasons why the last pip is slightly longer than all the rest.