also: oscillating weight, inertia (rotor)
A weight as heavy as possible, designed to wind a watch through the body movements of the wearer. The most common construction concept is either a central rotor or a micro-rotor (also: planetary rotor). The rotations of the rotor then wind the movement of the watch automatically. (Therefore: "automatic" watch).
There are single and bi-directional winding inertias. The first bi-directional winding mechanism of a watch was presented by Felsa with the Bidynator. Well known unidirectional winding calibres are the Valjoux 7750 and the Lemania 5100. The calibre Rolex 3135 (used for example in the Rolex Submariner) works by bi-directional winding.