We have its designer, Edmund Beckett Denison, to thank.  The clock mechanism, made to Denison's requirements by clockmaker Edward John Dent, was completed before the tower itself was finished. This meant Denison had time to experiment. 

Instead of using the escapement originally designed, Denison invented the double three-legged

gravity escapement. This method provides the best separation between pendulum and clock mechanism. Together with an enclosed, wind-proof box sunk beneath the clockroom, Big Ben's pendulum is well protected from snow, ice and pigeons on the clock hands, and keeps remarkably accurate time.

Originally, the time kept by Big Ben was to be telegraphed to Greenwich so the clock's performance could be compared with the exact time kept by the Royal Observatory. Big Ben's clock received a signal from the observatory once on every hour, allowing the operator in the clock room to note and make up for any error.

In the Astronomer Royal's report for that first year, he noted that "the rate of this clock may be considered certain to much less than one second per week".

If the clock were left to its own devices, it would drift very gradually and go out of time, but penny coins are added to or taken away from the clock's pendulum to fine-tune the accuracy. Even to this day, old pennies, phased out of British currency by decimalisation in 1971, are used.

By using halfpennies the effect is halved. This system has been very effective throughout the life of the clock. Unfortunately, it is no longer possible to compare the clock's rate with a signal from the observatory because the special telegraph line was destroyed by the Luftwaffe during the second world war. It was not considered worth the expense of renewing it, since after 75 years of daily comparison, the clock's accuracy had been demonstrated!

In fact, the clock rarely varies by more than two seconds a day, but usually it is exactly on time.

Big Ben today

The clock is wound and checked for accuracy three times a week. Although the going train (which drives the hands and controls the operation of both the chiming and the striking trains) has the capacity to run for about ten days, it is still wound at each inspection to allow for sickness or other unforeseen circumstances.

As a time signal is no longer received directly from the Royal Observatory, the operator checks the clock's performance against the telephone speaking clock by using a stopwatch and records any error, together with the barometric pressure. Coins can then be added to or removed from the pendulum if needed.

Despite running accurately throughout The Blitz, there have been a few glitches, including:

• 1962, when it slowed down on New Year's Eve 1962 due to heavy snow, causing it to chime in the New Year ten minutes late!

• 1976: the clock had its first and only major breakdown. The chiming mechanism broke due to metal fatigue on August 5 and was reactivated again on May 9, 1977. During this time BBC Radio 4 had to make do with the pips.

• 1997: it stopped on April 30, the day before the general election, and again three weeks later.