The Great Clock
The giant clock kept on time with old pennies.
2 · The question
The Great Clock in London is fine-tuned with old pennies. What does a penny do?
3 · Play first
The Great Clock is running slow. Add or take off pennies until it keeps time.
Running slow: −1.2 s a day, so 8.4 s behind after a week.
Too slow. Add 3 pennies: the weight sits higher, the pendulum acts shorter, and it speeds up.
4 · What's happening
- 1
Big Ben is the bell. The Great Clock is in the Elizabeth Tower, and a pendulum about 4.4 m long sets its pace, one beat every 2 seconds.
- 2
A shorter pendulum swings faster. A longer one swings slower.
- 3
On a small tray on the pendulum sit old pennies, from before 1971. Add one, and the pendulum's weight sits a touch higher, like a slightly shorter pendulum.
- 4
Each penny makes the clock gain about 0.4 seconds a day. Running slow? Add a penny. Running fast? Take one off.
5 · The maths
A pendulum's swing time depends on its length, not its weight. A beat of 2 s means 4 s there and back, which gives an effective length a bit under the 4.4 m of metal, because the weight is spread along it.
T = 2π√(3.97595 m ÷ 9.81)=4.000056 s
86,400 × (4 ÷ 4.000056 − 1) = -1.2 s a day
With the 4 pennies on the tray above. One penny moves L by only about 0.04 mm.
6 · Under the hood
Inside the real clock (for grown-ups)
Big Ben is the name of the Great Bell. The clock is the Great Clock, and its tower was renamed the Elizabeth Tower in 2012.
It is about 4.4 m long and beats once every 2 seconds. A full swing, there and back, takes about 4 seconds.
The coins are old pennies, made before the UK changed to decimal money in 1971. Clockmakers add or take them off to fine-tune the clock.
One penny moves the pendulum's effective length by only about 0.04 mm, far too small to see. That's why the drawing above exaggerates it. Over a day, it adds up to about 0.4 seconds.
7 · Try it yourself
Time a blu-tack pendulum
Hang a small weight on a string about 1 m long. Time 10 swings. Now press a blob of blu-tack onto the string well above the weight and time 10 swings again. Slide the blob up and down and see how the time changes. Tap each time you time a run.
Build it
A tabletop pendulum clock kit is on the list: a real pendulum with a little tray you can trim with coins.
Tell me when it's ready