The bell weighs 13.8 tonnes (13 tons 10cwt 99lb), with a striking hammer weighing 203.2kg (4cwt), and is tuned to E. There is delay of 5 seconds between strikes. The original tower designs demanded a 14 tonne bell to be struck with a 6cwt hammer. A bell was produced by John Warner and Sons, weighing 16 tons. However, this cracked under test in the Palace Yard. The contract for the bell was then given to the Whitechapel Bell Foundry, who re-cast the bell into the 13 ton bell used today. It too started to crack under the 6cwt hammer, and a legal battle arose. After two years of having the Great Bell out of commission, the 6cwt hammer was replaced with a lighter 4cwt hammer, and the bell itself was turned 90 degrees so the crack would not develop any further.
The belfry also houses four quarter bells which play the Westminster Chimes, derived from Handel's Messiah, on the quarter hours. The C note in the chime is repeated twice in quick succession, faster than the chiming train can draw back the hammers, so the C bell uses two separate hammers.
Big Ben is a focus of New Year celebrations in the UK, with radio and TV stations tuning to its chimes to welcome the 'official' start of the year. It can also be heard striking the hour before some news bulletins on BBC Radio 4 and the BBC World Service, a practice that began on December 31, 1923. ITN's "News at Ten" also used to begin with the famous chimes. These have since been dropped, but all ITV news bulletins now use a graphic based on the Westminster clock face.
The clock is famous for its reliability. This is due to its designer, the lawyer and amateur horologist Edmund Beckett Denison, later Lord Grimthorpe. As the clock mechanism, created to Denison's specification by clockmaker Edward John Dent, was completed before the tower itself was finished, Denison had time to experiment with the clock. Instead of using the deadbeat escapement and remontoire as originally designed, Denison invented the double three legged gravity escapement. This escapement provides the best separation between pendulum and clock mechanism. Together with an enclosed, wind-proof box sunk beneath the clockroom, the Great Clock's pendulum is well isolated from external factors like snow, ice and pigeons on the clock hands, and keeps remarkably accurate time.
The idiom of putting a penny on, with the meaning of slowing down, sprung from the method of fine-tuning the clock's pendulum by adding or subtracting penny-coins. Even to this day, only old pennies, phased out of British currency during the 1971 Decimalization, are used.
A 20-foot metal replica of the clock tower known as Little Ben, complete with working clock, stands on a traffic island close to Victoria Station. Several turret clocks around the world are inspired by the look of the Great Clock, including the clock tower of the Gare de Lyon in Paris and the clock tower of the Parliament of Canada in Ottawa.