The Great Clock

Accurate to a second, regulated with pennies

The Victorians wanted the most accurate public clock in the world. A lawyer designed it.

Big Ben is the bell; the Elizabeth Tower is the tower. Dates and figures come from the sources linked beside them; where sources disagree, we show each one.

Who would build it?

Barry was no clock specialist. He first asked Benjamin Lewis Vulliamy, the Queen’s Clockmaker, but other clockmakers such as Edward John Dent wanted the job, and disputes broke out. UK Parliament

In 1846 a competition was held to decide who would build the clock, refereed by the Astronomer Royal, Sir George Airy. UK Parliament

Airy wanted the most accurate turret clock in the world: the first stroke of each hour had to be right to within one second. UK Parliament

Until then, time had largely gone by the sun, so it varied by about 15 minutes across Britain. UK Parliament

The accuracy rules meant it took seven years before the designs were finalised. UK Parliament, key dates

The winning designs, in February 1852, were by Edmund Beckett Denison, a lawyer and amateur clock expert, so the chronometer maker Edward John Dent was appointed to build the clock. UK Parliament

Making it

The mechanism took two years to make and cost £2,500, which Parliament puts at about £285,000 today. UK Parliament

The clock mechanism was completed in 1854, years before the tower was ready for it. UK Parliament, key dates

The clock was installed in April 1859, once the building work was complete, and began keeping time on 31 May 1859. UK Parliament

To check it, the time from the Royal Observatory at Greenwich was sent to Westminster by telegraph wire. The clock passed. UK Parliament

The dials and hands

There are four clock dials, each made of 324 pieces of pot opal glass in a cast-iron frame. Parliament’s facts page gives the dials as 7 metres across, with hour figures 60 centimetres long. UK Parliament, facts and figures

Four o’clock is shown as IV, not the IIII found on many clocks. UK Parliament, 2021

The minute hands are copper sheet, 4.2 metres long and 100 kilograms each. They travel about 190 kilometres a year, roughly London to Weymouth. UK Parliament, facts and figures

The hour hands are gunmetal, 2.7 metres long and three times heavier, at 300 kilograms. UK Parliament, facts and figures

The mechanism

Parliament’s facts page says the mechanism weighs 5 tonnes, is made of cast iron, and is 4.7 metres long and 1.4 metres wide. UK Parliament, facts and figures

Wind on the huge hands could upset the timekeeping, so Denison designed the ‘Double Three-legged Gravity Escapement’, which compensates for outside forces on the mechanism. UK Parliament, facts and figures

Each beat of the pendulum lasts two seconds. UK Parliament, facts and figures

The same page gives the pendulum as 4.4 metres long and 310 kilograms, with a 203-kilogram bob made of concentric steel and zinc tubes. UK Parliament, facts and figures

The clock is fine-tuned with pre-decimal pennies: adding one makes the clock gain two-fifths of a second in 24 hours. UK Parliament, facts and figures

How heavy is the clock?

Is it right?

In 1930 an MP complained that Big Ben was three minutes slow compared with every other clock. The minister replied that in 290 days the error had passed one second on only eight, and that “all the others are wrong and that Big Ben is right”. UK Parliament

The Great Clock was the largest and most accurate clock in the world when it was built, and it is still the biggest striking and chiming turret clock in the UK. UK Parliament

When it stops

Parliament lists some long Great Clock stoppages: two months in 1934, six months in 1956, and six weeks in 2007, the longest since 1990. In 2005 it stopped for two days so the brake shaft could be inspected. UK Parliament, facts and figures

On 5 August 1976 a mechanical failure badly damaged the clock: the weights spiralled out of control down the weight shaft and the mechanism blew apart. Big Ben was silent for nearly nine months, back in time for the Queen’s Silver Jubilee visit to Westminster Hall in May 1977. UK Parliament, key dates

In 2007 Big Ben and the quarter bells were silenced from 11 August to 1 October for maintenance on the clock. UK Parliament, key dates

There are more than 2,000 clocks on the Parliamentary Estate, and Parliament’s clock mechanics change them all, the Great Clock included, when the clocks go forward and back. UK Parliament, 2021

How the clock was taken apart and rebuilt in 2017–22 is on the restoration page.

Next: the tower →