Pendulum clock
Before the pendulum, the best clocks drifted by a quarter of an hour a day. Christiaan Huygens hung a swinging weight on them, and the error fell to fifteen seconds.
- 1656Huygens builds the first pendulum clock
- ≈ 993 mmthe length of a seconds pendulum
- ±43 s a daywhat one millimetre of length is worth
An open clock
On the right is a clock without its case: a brass plate, wheels, the anchor, a weight and the pendulum behind the movement. The hands show your real time, and the seconds hand jumps at every beat.
This was one of the first clocks that could fairly be called accurate.
Time depends only on length
The story goes that Galileo, still a student, noticed in Pisa Cathedral that a lamp swung with the same period however wide its swing. A pendulum's period depends only on its length and on gravity.
T = 2π · √(L / g)A pendulum 993 mm long makes one swing a second, which is why it is called a seconds pendulum.
A wheel released tooth by tooth
Above the escape wheel rocks the anchor. Its crutch is linked to the pendulum: at each swing one pallet releases the wheel and the other at once catches the next tooth. That is the tick.
At each release the tooth gives the pallet a slight push, and the pendulum gets back exactly the energy it loses to friction. The anchor was invented in England around 1670, and with it a pendulum could swing through a small arc, about 4°.
A week of energy
The clock is wound by raising the weight. It sinks slowly on its line and turns the barrel. Gravity is constant, so the weight pulls just as hard at the end of the run as at the start, unlike a spring.
From seconds to hours
Three pairs of wheels turn the slow rotation of the barrel into the fast rotation of the escape wheel. The tooth counts in the drawing are the real ones.
144/12 · 80/8 · 48/8 = 720The 30-tooth escape wheel moves half a tooth per beat, so it turns once a minute. The centre wheel turns once an hour, the barrel once in 12 hours.
A skeleton chapter ring
The minute hand sits right on the centre wheel's arbor. The hour hand is driven through a small 12-to-1 gearing, and the seconds hand is mounted on the escape wheel's arbor.
In layers, like a sandwich
The wheels don't lie in one plane but one in front of another. Each arbor is held by the back plate and a front bridge. Press "Take apart" on the drawing to put the movement back together.
The nut under the bob
If the clock gains, the bob is lowered with the nut; if it loses, the bob is raised. One millimetre of length changes the rate by about 43 seconds a day.
We have shortened the pendulum by 11 mm, and the readout in the drawing's corner shows how much the clock will now gain in a day. Move the pointer up and down over the drawing to turn the nut yourself.