An automatic watch is a mechanical watch that winds itself. There is no battery and no electronics: all of its energy comes from the movement of your wrist.
The rotor
A half-moon weight called the rotor sits on the back of the movement and turns freely as you move. Through a few gears it winds the mainspring. On watches with a see-through case back you can watch it spin.
The mainspring
The mainspring is a long strip of metal coiled inside a barrel; the more it is wound, the more energy it stores. How long the watch keeps running off the wrist is its power reserve, usually 38 to 80 hours. Tissot's Powermatic 80, for example, runs for up to 80 hours.
Escapement and balance
The escapement lets the spring's energy out in small, even steps, and the balance wheel with its hairspring swings back and forth to set the beat. Many modern movements beat 28,800 times an hour, eight ticks a second, which is why the seconds hand sweeps smoothly.
If you have not worn it for a few days
- If the crown screws down, unscrew it first.
- Without pulling it out, turn the crown 20 to 30 times to wind the spring; most automatic movements can also be wound by hand.
- Set the time and date, screw the crown back in and put the watch on.
A typical automatic watch gains or loses a few seconds a day. COSC-certified chronometers stay within −4 to +6 seconds a day.