What is a watch rotor?

The watch rotor is an oscillating weight that pivots inside a self-winding (automatic) watch. The movement of the wearers arm causes the rotor to pivot on its staff; this is attached to the winding mechanism. The motions created by the wearer’s arms are converted into circular motions that feed the rotor via a series of reverser and reducing gears to wind the mainspring.

Depending on the type of watch the rotor can swing in one or two directions with more advance rotors having two ratchets to wind the mainspring during clockwise and anti-clockwise rotor motions.