There are six pips (short beeps) in total. Five 1 kHz pips are broadcast starting at 59 minutes and 55 seconds, lasting a tenth of a second each. A final pip, lasting half a second, is played at at the start of the new hour. When a year has a leap second, seven pips are used. The leap second is also the explanation for the sixth pip being longer than the others. This is so that it is always clear which pip is on the hour, especially where there is an extra pip that some people might not be expecting. At the start of the 6 o'clock evening and midnight news the pips are replaced by the chimes of Big Ben, where the first chime represents the start of the hour.
Officially called the Greenwich Time Signal, the pips have been broadcast since 1924, and were the idea of the Astronomer Royal Sir Frank Watson Dyson and head of the BBC John Reith. The pips were originally controlled by two mechanical clocks located in the Royal Greenwich Observatory that had electrical contacts attatched to the pendulums. Two clocks were used in case of a breakdown. These sent a signal each second to the BBC, who converted them to the audible oscillatory signal that is broadcast.
The BBC compensates for the time delay in both broadcasting and receiving equipment, as well as the time for the actual transmission. The pips are timed so that they are accurate for people living 100 miles from Broadcasting House. The pips are therefore most exact for people living in Leicester, Norwich, Swindon and Calais in France.