Structure
There are six pips (short beeps) in total, which occur on the 5 seconds leading up to the hour and on the hour itself. Each pip is a 1 kHz tone (about half way between musical B5 and C6), which, for the first five, last a tenth of a second, while the final pip lasts half a second. The actual moment when the hour changes – the "on-time marker" – is at the very beginning of the last pip.
When a leap second occurs (exactly one second before midnight), it is indicated by a seventh pip. In this case the first pip occurs at 23:59:55 (as usual) and there is a sixth short pip at 23:59:60 (the leap second) followed by the long pip at 00:00:00. The leap second is also the explanation for the final 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. Before leap seconds were conceived the final pip was the same length as the others. "Negative" leap seconds can also be used to make the year shorter, but in practice this has never happened.
Although normally broadcast only on the hour by BBC domestic radio, BBC World Service use the signal at other times as well. The signal is generated at each quarter hour and has on occasion been broadcast in error.
Read more about this topic: Greenwich Time Signal
Famous quotes containing the word structure:
“A committee is organic rather than mechanical in its nature: it is not a structure but a plant. It takes root and grows, it flowers, wilts, and dies, scattering the seed from which other committees will bloom in their turn.”
—C. Northcote Parkinson (1909–1993)
“Women over fifty already form one of the largest groups in the population structure of the western world. As long as they like themselves, they will not be an oppressed minority. In order to like themselves they must reject trivialization by others of who and what they are. A grown woman should not have to masquerade as a girl in order to remain in the land of the living.”
—Germaine Greer (b. 1939)
“Man is more disposed to domination than freedom; and a structure of dominion not only gladdens the eye of the master who rears and protects it, but even its servants are uplifted by the thought that they are members of a whole, which rises high above the life and strength of single generations.”
—Karl Wilhelm Von Humboldt (1767–1835)