How Does an Automatic Watch Work Without a Battery?

An automatic watch has no battery, no chip and no charger, yet it can keep good time for decades. The whole thing runs on the movement of your wrist. Here is how, in five parts.

1. The rotor: your wrist is the power source

Turn an automatic watch over and, if it has a display caseback, you will see a half-moon weight. That is the rotor. Every time your wrist moves, it swings around its centre. Through a small set of gears, that motion winds the mainspring. It is why an automatic is also called a self-winding watch.

2. The mainspring: where the energy is stored

The mainspring is a long, thin coiled spring inside a small drum called the barrel. The rotor tightens it, and as it slowly unwinds it releases energy at a steady pace. A fully wound mainspring typically runs the watch for 38 to 42 hours on the movements we use. That figure is the power reserve.

You can't overwind an automatic: a slipping clutch in the barrel lets the spring slide once it is full. Most automatics can also be wound by hand with the crown, which is the quickest way to start one that has stopped.

3. The gear train: carrying the energy

A series of gears carries that energy from the barrel to the hands, stepping it down so the seconds, minutes and hours each turn at the right speed.

4. The escapement: letting time out in steps

If nothing held it back, the spring would unwind in seconds. The escapement stops that. A small escape wheel is locked and released, tooth by tooth, by a lever called the pallet fork. Each release lets the gears advance a tiny step. That is the tick you hear.

5. The balance wheel and hairspring: the heart

The pallet fork is controlled by the balance wheel, which swings back and forth on a hairspring finer than a human hair. Its rhythm is the watch's heartbeat. The Swiss Sellita SW200-1 in our Elemental beats at 28,800 vibrations an hour, or 4 Hz: eight small steps every second, which is why its seconds hand seems to glide rather than jump.

Why the seconds hand sweeps

A quartz watch moves its seconds hand once a second. A mechanical watch moves it several times a second, so it looks like a smooth sweep. The higher the beat, the smoother the sweep.

What this means for you

• No battery to change, ever. Wear it and it runs.
• It will stop if you leave it off longer than its power reserve. That is normal; wind it and set it.
• It needs a service every five to seven years to clean and re-oil the movement. Looked after, it can outlive you.
• It is less accurate than quartz, by a few seconds a day. That is the price of a machine with no battery that you can pass on.

Next: how to wind and set an automatic watch for the first time.