An automatic watch is one where the mainspring is wound by inertia and the movements of the wearer. Typically, a centrally mounted rotor spins around and, via reversers or magic lever, winds the mainspring.
Here's the rotor on my ETA 2824:
There are variations:
- Early automatics used edge-mounted weights and gear cages. Citizen did some of these; they were less efficient.
(Image credit: WatchUSeek, a Citizen Jet Parawater)
- Omega and others made so-called 'bumper' movements that swung through an arc and stopped by a spring.
- Micro-rotors like the Universal Geneve from the 1960s and the current Chopard/Parmigiani-Flurier 1.96.
See ETA 2824 for a typical current automatic movement.