Stop 82 of 109 · A Penny on the Pendulum
1859 · London, Palace of Westminster
How a Handful of Old Pennies Keeps Big Ben on Time
The Great Clock of Westminster is fine-tuned not with modern electronics but with pre-decimal pennies stacked on its pendulum, and Big Ben, strictly speaking, isn't even the clock.
First, a correction the tower itself doesn't mind repeating: Big Ben is not the clock, and it isn't the tower either. It's the name of the Great Bell, the one that strikes the hour. The tower that houses it all was simply the Clock Tower until 2012, when it was renamed the Elizabeth Tower.
The clock beneath the bell, though, has its own quietly ingenious trick. Its huge pendulum, 4.4 metres long and weighing over 300 kilograms, swings once every two seconds. To keep it accurate, the clock's keeper doesn't adjust a screw or a spring: they add or remove old pre-decimal pennies from a small tray resting on top of the pendulum.
Each penny added raises the pendulum's centre of mass just slightly, which speeds up its swing by about two-fifths of a second every day. It's a crude-sounding method for one of the most famous clocks in the world, but it works: engineers can nudge the Great Clock's rate with a precision most electronic systems would envy, using coins from a currency Britain stopped minting decades ago.
The principle hasn't disappeared from watchmaking, either. Many fine mechanical watches are still regulated the same way in miniature, with tiny weights fitted around the rim of the balance wheel, the small wheel that swings back and forth to set a watch's pace, doing on a wristwatch scale exactly what pennies do on Big Ben's pendulum.