Inside the clock: how a pendulum clock works

Watch the parts of a weight-driven pendulum clock move together. Slow it down, step it one tick at a time, and tap any part to see what it does.

The whole clock in one minute

A mechanical clock needs two things: a source of power and something that keeps a steady beat. In this clock the power comes from a falling weight. Its pull turns a series of gears, called the train, and the last gear, the escape wheel, would spin away in seconds if nothing held it back.

The steady beat comes from the pendulum. As it swings, it rocks the anchor, which lets the escape wheel move forward by one tooth at a time, with a tick each time. Each released tooth also gives the anchor a small push that keeps the pendulum swinging. So the weight provides the energy, the pendulum sets the pace, and the escapement connects the two. The gears simply count the ticks and turn the hands.

What this model leaves out

This is a simplified teaching model, drawn to be easy to follow rather than to scale. Real movements put the wheels between two plates, use small toothed pinions to drive each wheel, and often have more wheels, a striking or chiming train, and a different number of teeth. The motion work that turns the hour hand sits behind the dial and is not drawn. Many clocks also use a mainspring instead of a weight; the same escapement idea still applies.

Want the full story? Read how the pendulum clock was invented and how escapements work, look up terms in the clock glossary, or learn how to wind a clock.