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World's largest 4-dial clock

This is actually the youngest of all the clocks in these articles on important clocks but it surely deserves inclusion, not just because it's the largest four-dial clock in the World but because it's an iconic symbol of London, freedom and democracy.

'Big Ben' is the name widely used to mean the clock or the whole Palace of Westminster tower overlooking the River Thames in London. But virtually everyone actually understands 'Big Ben' to be the largest of the five bells in the clock tower, on which the hour is struck. Well, that's not quite right either. Big Ben, after Sir Benjamin Hall, was the name given to the original 16 ton bell cast by Warners in Stockton on Tees in August 1856. It was the largest bell even made in Britain but it cracked during tests before it could be lifted into position. No one accepted the blame.

A replacement bell weighing 13½ tons was cast by George Mears in London in April 1858 at the Whitechapel Bell Foundry, which itself dates back to 1570. In 1752, a smaller bell cast there became the Liberty Bell. That cracked the first time it was struck. and unfortunately, the replacement Big Ben also cracked two months after installation at the point where the hammer struck. Sir George Airy (Astronomer Royal) solved the problem by rotating the bell 90 degrees, cutting a square at the end of the crack to stop it spreading and installing a lighter hammer. It is still used today, but the hourly strike has a slightly odd tone.

The five-ton, cast iron three train movement was constructed by Edward Dent and finished in 1858 by his stepson, Frederick. It cost £2,500 and measures 4.7 metres by 1.4. While waiting for the Tower to be finished, Frederick changed the original deadbeat escapement to a three-legged gravity escapement invented by an amateur clockmaker, Edmund Dennison (Lord Grimthorpe). His design compensated for wind pressure on the hands and allowed the pendulum to be disconnected quickly to avert damage if a mechanical problem arose.

At 4.4 metres long, the pendulum is four times the 'seconds' pendulum of a longcase clock and takes 2 seconds to swing each way. Made from concentric tubes of zinc and steel, it weighs over 300kgs alone. To ensure accuracy, it is protected from the wind by its own brick-built enclosure and it's perfectly true that a pile of old pennies sits on top of the bob, each one serving to raise the centre of mass sufficiently to advance the time by 0.4 seconds a day.

Each cast iron dial measures 7 metres in diameter and contains 312 pieces of glass, which were replaced in 1956. They are cleaned every five years. Note that unlike almost every other clock you'll ever see, the 4 o'clock marker is the conventional Roman IV, whereas IIII is traditionally used on dials to maintain some sort of balance with the VIII opposite.

The four original 4.2 metre minute hands were cast iron and proved to be too heavy to turn so lighter copper ones were made weighing 100kgs each. The four shorter 2.7 metre hour hands are gunmetal and weigh 300kgs.

The twice-yearly routine of switching between BST and GMT begins just after 9pm on Saturday evening. The clock then remains silent and in darkness until 2am while the hands are advanced to midnight, which takes two minutes. There is even a prescribed order for switching the dial lights back on (West, South, East then North). At the same time, a team of horologists adjust the other 2,000 clocks in the house!

If you'd like to hear Big Ben strike 12, run the movie in the bottom image; you'll need QuickTime plugin if you don't have it.

CLICK HERE if you'd like to see an animation of how the clock works.


  • Big Ben in London has four dials
  • Big Ben - the bell
  • Big Ben's strike train
  • Big Ben's chime train