How an automatic watch works

Reach for your morning coffee. Pull on a jacket. Close a door. Inside your watch, a small weight responds to these ordinary gestures. Before you have thought about the time, you have already given the movement a little energy.

An automatic watch winds its mainspring using movement of the wrist. The spring powers a train of gears; a balance and escapement regulate their advance. In a conventional automatic mechanical watch, this happens without a battery. Here is the journey, from the first swing of the rotor to the next step of the seconds hand.

The raspberry part in our opening drawing is the rotor, also called the oscillating weight. Its off-centre mass lets it swing around a bearing as the watch moves. Gears connect it to the winding system, which transfers that motion to the mainspring.

The illustrated ETA 2824-2 winds in both directions. Other automatic calibres wind in just one, so a rotor that spins freely one way is not necessarily faulty. Through a transparent caseback, the broad moving weight is often the first component you notice.

Look beyond it and you may spot another moving wheel. That is the balance, much smaller and much faster. The rotor gathers energy from your day. The balance helps decide the rhythm at which the watch spends it.

Your wrist does not drive the hands directly. Between the winding system and the gear train sits a long, flat strip of spring metal, coiled inside a drum called the barrel. This mainspring is the movement's energy store.

Winding tightens the spring around its central arbor. As the spring releases its stored energy, it turns the barrel. Teeth around the barrel drive the next wheel, passing power into the gear train. The gears transmit motion at the ratios needed to display seconds, minutes and hours.

In the drawing, follow the tightly packed raspberry lines through the openings in the metalwork. They are the visible edges of one coiled ribbon, not separate rings. This skeleton movement makes the spring visible; in many watches, a solid barrel lid conceals it.

You supply the movement.
The spring saves a little of it for later.

Winding adds energy. Running uses it. Both can happen while you wear the watch, and running can continue after the rotor becomes still. Automatic mainsprings commonly use a slipping attachment at the barrel wall so the winding system can continue operating when the spring is fully wound.

There are two very different springs in this story. The mainspring powers the movement. The much finer hairspring, attached to the balance, helps establish its rhythm. Remember that distinction as we move closer.

Now look for a wheel that turns one way, stops, and returns. Unlike the rotor, the balance does not normally make continuous revolutions. It oscillates around its central staff, working with the fine spiral attached to it.

As the balance moves away from its resting position, the hairspring provides a restoring force. The wheel passes back through the centre and swings in the other direction. The balance's inertia and the spring's properties establish the natural rhythm of this oscillator.

Our drawing highlights the balance rim and the visible hairspring beneath the central support. The broad circular metal surface on top is part of the supporting assembly: its concentric machining marks are not the hairspring. The actual spring is the fine set of separated curves below it.

The balance loses energy to friction and other resistance, so it needs small impulses to keep oscillating. Those arrive through the escapement. Think of a pendulum receiving a gentle, well-timed push: the push sustains the motion while the oscillator sets the pace.

For the ETA 2824-2 in our opening, that pace is 4 Hz: four complete back-and-forth oscillations per second. Each full oscillation contains two beats, giving eight beats each second, or 28,800 vibrations per hour. Other calibres run at other frequencies; this is one example.

A wound spring and a gear train would run down rapidly without a way to control their motion. The lever escapement connects that source of power to the oscillator. Its job has two sides: restrain the gear train, and keep the balance moving.

The escape wheel receives power from the train. Its specially shaped teeth meet two small pallet stones carried by a lever, also called the pallet fork. One pallet locks the wheel while the balance continues through most of its swing.

As the balance returns towards the centre, its impulse jewel engages the fork and moves the lever. The locked tooth is released. As the escape wheel advances, it transfers energy through the lever to the balance; the opposite pallet then locks the wheel again. The sequence repeats on the return swing.

In the illustration, the highlighted teeth and two pallet stones appear between the bridges. Some connecting parts are hidden in the original photograph and remain hidden in the drawing. These are observed details of a real lever mechanism, not an invented exploded assembly.

That repeated locking and release is behind the familiar tick. On many mechanical watches, the seconds hand appears to sweep because its small advances happen several times a second. With the 4 Hz example above, there are eight beats in the time it takes to say “one”.