Device for automatically winding a watch. In practice, a half-moon-shaped piece which turns freely in both directions and uses the force of gravity to wind the mainspring. Some rotors only turn in one direction or less than 360°. Each time it moves, it advances the ratchet-wheel, which keeps the spring wound. In wristwatches, the rotor is generally made of brass, but, as its weight is a determining factor in keeping the watch properly wound, other materials are also used: 18K, 21K, 24K gold or platinum, for example, in top quality watches.