Turret clock with double three-legged gravity escapement · England
The Great Clock of Westminster ('Big Ben', properly the bell)
Verified'Big Ben' properly refers to the Great Bell; the phrase is very widely used for the whole clock and tower, and this popular usage should be flagged rather than silently repeated as the technically correct name.
Technical facts
- Escapement
- Denison's double three-legged gravity escapement
- Pendulum
- Approximately 4 metres long, beating at roughly two-second intervals, with the well-known coin-adjustment method for fine regulation
- Movement maker
- E. J. Dent & Co., completed after Dent's death by his stepson Frederick Dent
Historical significance
- 'Big Ben' properly refers to the Great Bell; the phrase is very widely used for the whole clock and tower, and this popular usage should be flagged rather than silently repeated as the technically correct name.
- Denison's double three-legged gravity escapement isolates the pendulum from variations in the driving force caused by wind pressure on the massive external hands, a design innovation of lasting influence on precision turret clocks.
- The clock has undergone significant conservation work in the 21st century; researchers should check the relevant Parliamentary/UK Parliament conservation programme documentation for the current mechanical state rather than assuming an unaltered 1859 mechanism.
Background
The Great Clock was designed by the lawyer and amateur horologist Edmund Beckett Denison and George Airy, the Astronomer Royal; it is a striking clock with five bells. Construction was entrusted to the clockmaker Edward John Dent; after his death in 1853, his stepson Frederick Dent completed the work in 1854. Because the tower was not finished until 1859, Denison replaced the deadbeat escapement and remontoire of the original design with his double three-legged gravity escapement, which best separates the pendulum from the clock mechanism and so lessens the effects of rain, wind and snow on the dials.
Upon its completion, the mechanism was noted for being the largest and most accurate four-faced striking and chiming clock in the world. The clock is regulated by adding or removing pre-decimal pennies on the pendulum, which changes the rate at which the pendulum swings.
Written from the encyclopedia entries listed under Sources; each statement was checked against the cited text.
Construction and mechanism
- The four clock dials are 22.5 feet in diameter and are constructed from cast iron sections with opalescent glass.
- Adding or removing one penny on the pendulum changes the clock's rate by 0.4 seconds per day.
- The clock keeps time to within a few seconds per week and is hand wound three times a week, each winding taking about 1.5 hours.
Later history
- Denison never patented his escapement, and it quickly became the standard on all new high-quality tower clocks.
- When completed, the dials and hands were Prussian blue; they were painted black in the 1930s to disguise the effects of air pollution.
- In 2009 three of the pennies on the pendulum were replaced with a £5 coin minted for the 2012 Summer Olympics.
- The clock and tower were renovated between 2017 and 2021, when the original colour scheme was reinstated and the bells stayed silent with a few exceptions.
Photographs
Photographs are illustrative examples of the type, not necessarily the exact object described. Verify details against the source file page.
Sources & references
- Big Ben, Wikipedia (English), retrieved 2026-10-28 (text CC BY-SA 4.0)
- UK Parliament, published history and conservation information on the Great Clock
- Antiquarian Horological Society, articles on Denison's gravity escapement
Where sources disagree, the disagreement is stated rather than resolved silently.