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Since 1924 the BBC have used a time signal of six short pips to mark the hour. They begin at the 55-second instant, and the new minute begins at the beginning of the sixth pip, which is somewhat longer.

Not every hour: on the music station Radio 3 they only use them when they are about to have a short news bulletin, so that's mainly in the morning, and the main news on Radio 4 is introduced by the powerful chimes of Big Ben. But the "pips" as they are called, officially the "Time Signal", are very distinctive and instantly recognized.

The were devised by the astronomer royal Sir Frank Dyson in 1923, in consultation with the head of the BBC, John Reith, and the horologist and inventor Frank Hope-Jones. They were created by Dent regulators built in 1874, housed at the Royal Observatory in Greenwich. For the New Year of 1924, Big Ben was broadcast for the first time, as part of the same arrangement. Electrical contacts with the pendulum of the Dent clocks (two were used so there was a back-up) sent a signal to Broadcasting House in London where they were converted to sound by an oscillatory valve. The frequency is about 1 kHz.

They moved to observatories in Abinger in 1939 then Herstmonceux in 1957. In 1990 the BBC took over the task themselves, and now produce their time signal by GPS in coordination with the National Physical Laboratory.

When a leap second is added to a day to bring UTC (formerly called Greenwich Mean Time) into line with the earth's orbit, a seventh pip is used: the six short ones are at seconds 55 to 60, and the longer one still begins the new minute, at second 61 (= 00).

More detail at:
which is a news story from February 1999, when the Dent clocks were taken out of retirement and displayed at Greenwich for the 75th anniversary of the pips.

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