The tower was raised as a part of Charles Barry's design for a new palace, after the old Palace of Westminster was destroyed by fire on the night of 22 October 1834. However, although Barry was the chief architect of the palace, he turned to Augustus Pugin for the design of the clock tower, which resembles earlier Pugin designs, including one for Scarisbrick Hall. The design for Big Ben was, in fact, Pugin's last design before his final descent into madness and death, and Pugin himself wrote, at the time of Barry's last visit to him to collect the drawings: "I never worked so hard in my life for Mr Barry for tomorrow I render all the designs for finishing his bell tower & it is beautiful" (Rosemary Hill, God's Architect: Pugin & the Building of Romantic Britain (2007) p 482). The tower is designed in Pugin's celebrated Gothic revival style, and is 96.3 metres (315.9 ft) high.

The first 61 metres (200 ft) of the structure is the Clock Tower, consisting of brickwork with stone cladding; the remainder of the tower's height is a framed spire of cast iron. The tower is founded on a 15 metres (49 ft) square raft, made of 3 metres (10 ft) thick concrete, at a depth of 7 metres (23 ft) below ground level. The four clock faces are 55 metres (180 ft) above ground. The interior volume of the tower is 4,650 cubic metres (164,200 cubic feet).

Due to ground conditions present since construction, the tower leans slightly to the north-west, by roughly 220 millimetres (8.66 in) at the clock face, giving an inclination of approximately 1/250. Due to thermal effects it oscillates annually by a few millimetres east and west.

Clock faces

The clock faces are large enough to have once allowed the Clock Tower to be the largest four-faced clock in the world, but have since been outdone by the Allen-Bradley Clock Tower in Milwaukee, Wisconsin. The builders of the Allen-Bradley Clock Tower did not add chimes to the clock, so the Great Clock of Westminster still holds the title of the "world's largest four-faced chiming clock." The clock mechanism itself was completed by 1854, but the tower was not fully constructed until four years later in 1858.

The clock and dials were designed by Augustus Pugin. The clock faces are set in an iron framework 7 metres (23 ft) in diameter, supporting 312 pieces of opal glass, rather like a stained glass window. Some of the glass pieces may be removed for inspection of the hands. The surround of the dials is heavily gilded. At the base of each clock face in gilt letters is the Latin inscription: "DOMINE SALVAM FAC REGINAM NOSTRAM VICTORIAM PRIMAM", which means 'O Lord, keep safe our Queen Victoria the First'.

The clock became operational on 7 September 1859.

During World War II, the Palace of Westminster was hit by German bombing, destroying the Victorian House of Commons and causing damage to two of the clockfaces as well as sections of the tower's steeped roof.

The Great Bell

The main bell, officially known as the Great Bell, is the largest bell in the tower and part of the Great Clock of Westminster. The bell is better known by the nickname Big Ben, which is almost universally mistakenly applied to the Clock Tower.

The name Big Ben was given to a 14.5 tonne (16 ton) hour bell, cast on 10 April 1856 in Stockton-on-Tees by Warner's of Cripplegate. The bell was never officially named, but the legend on it records that the commissioner of works, Sir Benjamin Hall, was responsible for the order. Another theory for the origin of the name is that the bell may have been named after a contemporary heavyweight boxer Benjamin Caunt. It is thought that the bell was originally to be called "Victoria" or "Royal Victoria" in honour of Queen Victoria, but that an MP suggested the nickname during a Parliamentary debate; the comment is not recorded in Hansard.

Since the tower was not yet finished, the bell was mounted in New Palace Yard but the bell cracked under the striking hammer, and its metal was recast at the Whitechapel Bell Foundry as the 13.76 tonne (13.54 ton (long), 15.17 ton (short)) bell, which stands at a height of 2.2 metres with a diameter of 2.9 metres, and it is still in use today. The new bell, which chimes on A, was mounted in the tower alongside four quarter-hour bells, the ring of bells that ring the familiar changes.

Other Bells

Along with the main bell, the belfry houses four quarter bells which play the Westminster Quarters on the quarter hours. The four quarter bells are G sharp, F sharp, E, and B (see note). They play a 20-chime sequence, 1-4 at quarter past, 5-12 at half past, 13-20 and 1-4 at quarter to, and 5-20 on the hour. Because the low bell (B) is struck twice in quick succession, there is not enough time to pull a hammer back, and it is supplied with two wrench hammers on opposite sides of the bell.

Similar turret clocks

A 6 metre (20 ft) metal replica of the clock tower, known as Little Ben and 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 Peace Tower of the Canadian Parliament Buildings in Ottawa.

A clock tower similar to Big Ben is the Joseph Chamberlain Memorial Clock Tower of the University of Birmingham, England. Often referred to as "Old Tom" or "Old Joe", it is around three quarters of the size of Big Ben. Its four faces are each seventeen feet in diameter.

Baby Big Ben is the Welsh version of Big Ben at the pierhead in Cardiff. Its mechanism is almost identical to the one which powers the Big Ben clock in London.

There are other replicas, one of the finest is a two-third exact replica of the movement made by Dent located in the Queens Royal College Trinidad. There is another in Zimbabwe.

The 47 metre tower of the City Hall in Pietermaritzburg, South Africa, contains a quarter chime Westminster tower clock and carillon manufactured by Gillett and Johnston of Croydon. Its four faces are each three metres in diameter.

Reliability

The clock is famous for its reliability. This is due to the skill of 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. 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, sprang from the method of fine-tuning the clock's pendulum. The pendulum carries a small stack of old penny coins. Adding or subtracting coins has the effect of minutely altering the position of the bob's centre of mass, the effective length of the pendulum rod and hence the rate at which the pendulum swings. Adding or removing a penny will change the clock's speed by 2/5th of one second per day.

Despite heavy bombing the clock ran accurately throughout the Blitz. It slowed down on New Year's Eve 1962 due to heavy snow, causing it to chime in the new year 10 minutes late.

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

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

On Friday, 27 May 2005, the clock stopped ticking at 10.07 pm, possibly due to hot weather (temperatures in London had reached an unseasonal 31.8 °C (90 °F). It resumed keeping time, but stalled again at 10.20 pm and remained still for about 90 minutes before starting up again.

On 29 October 2005, the mechanism was stopped for approximately 33 hours so that the clock and its chimes could be worked on. It was the lengthiest maintenance shutdown in 22 years.

There were other short stoppages but the practice of the publicity department of the Houses of Parliament to attribute problems to weather and other reasons outside of their control makes it difficult to be sure why. Ex employees of Thwaites & Reed who looked after the clock for 30 years say problems were caused by a major overhaul for the millennium being shelved and never done. Thwaites & Reed say they have the exact details of what was needed, but they seem reluctant to make public their records even though their older records are on loan to the Guildhall Library in London for everyone to see.

Big Ben's "Quarter Bells" were taken out of commission for four weeks starting at 0700 hrs GMT on 5 June 2006, as a bearing holding one of the quarter bells was damaged from years of wear and needed to be removed for repairs. During this period, BBC Radio 4 broadcast recordings of British bird song followed by the pips in place of the usual chimes.

On August 11, 2007, Big Ben went silent and temporarily also stopped keeping time for maintenance that lasted 1 month. The bearings that help sound the chime on each hour were replaced, for the first time since installation. During the maintenance works, the clock was not driven by the original mechanism, but by an electric motor. Once again, BBC Radio 4 had to make do with the pips during this time.

Culture

The Clock Tower is a focus of New Year celebrations in the United Kingdom, with radio and TV stations tuning to its chimes to welcome the start of the year. Similarly, on Remembrance Day, the chimes of Big Ben are broadcast to mark the 11th hour of the 11th day of the 11th month and the start of two minutes' silence.

For years ITN's "News at Ten" began with an opening sequence which featured the Clock Tower and Big Ben with the chimes punctuating the announcement of the news headlines. The Big Ben chimes are still used today during the headlines and all ITV News bulletins use a graphic based on the Westminster clock face. Big Ben can also be heard striking the hour before some news bulletins on BBC Radio 4 (6pm and midnight, plus 10pm on Sundays) and the BBC World Service, a practice that began on 31 December 1923. The chimes are sent in real time from a microphone permanently installed in the tower and connected by line to Broadcasting House.

Big Ben can be used in the classroom to demonstrate the difference between the speed of light and the speed of sound. Someone visiting London who stands at the bottom of the clock tower will hear the chimes of Big Ben approximately one-sixth of a second later than the bell being struck (assuming a bell height of 55 metres). However, using a microphone placed near the bell and transmitting the sound to a far away destination by radio (for instance New York City or Hong Kong), that location will hear the bell before the person on the ground. In fact, if the recipient were to echo the sound back to the observer on the ground, the bell would be heard on the radio before the natural sound reached the observer. (Example: New York City is 5,562 kilometres (3,456 mi) from London, and radio waves will reach New York in 0.018552 seconds; round trip is 0.037105 seconds, compared with 0.1616 seconds for the natural sound to reach the ground.)

Londoners who live an appropriate distance from the Clock Tower and Big Ben can, by means of listening to the chimes both live and on the radio or television, hear the bell strike thirteen times on New Year's Eve. This is possible due to what amounts to a one strike offset between live and electronically-transmitted chimes by virtue of a combination of digital coding and decoding and satellite transit delay. Guests are invited to count the chimes aloud as the radio is gradually turned down.

An image of the clock tower was also used as the logo for London Films.

In Japan, Big Ben's chime is used as the chime for school bells from kindergarten to university.

In 2005, a terrorist manual was found in the home of Abu Hamza al-Masri, marking Big Ben, the Statue of Liberty and the Eiffel Tower as terrorist targets. In his trial at The Old Bailey in 2006 he denied all knowledge of their being targets.