What is used to calibrate Big Ben the famous clock of London’s Palace of Westminster?

Big Ben is the nickname for the Great Bell of the clock at the north end of the Palace of Westminster in London, and often extended to refer to the clock and the clock tower. The tower is officially known as Elizabeth Tower, renamed to celebrate the Diamond Jubilee of Elizabeth II in 2012; previously it was known simply as the Clock Tower.

The clock's movement is famous for its reliability. The designers were the lawyer and amateur horologist Edmund Beckett Denison, and George Airy, the Astronomer Royal. 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. The pendulum is installed within an enclosed windproof box beneath the clockroom. It is 13 feet (4.0 m) long, weighs 660 pounds (300 kg), suspended on a strip of spring steel 1/64 inch in thickness, and beats every 2 seconds. The clockwork mechanism in a room below weighs 5 tons. On top of the pendulum is a small stack of old penny coins; these are to adjust the time of the clock. Adding a coin has the effect of minutely lifting the position of the pendulum's centre of mass, reducing the effective length of the pendulum rod and hence increasing the rate at which the pendulum swings. Adding or removing a penny will change the clock's speed by 0.4 seconds per day.