The rotor is the oscillating mass that converts wrist movement into mechanical energy to wind the mainspring of an automatic watch. It is not a passive component: its geometry, material, and position within the movement directly affect charging efficiency, case thickness, and what can be seen through the sapphire caseback.
The classical rotor, known as the central rotor, is a half-moon shape that rotates 360° above the plate, covering approximately half of the movement. It is the most widespread solution because it balances production cost, efficiency, and robustness. I see thousands of them at the bench: in steel, tungsten, 22K gold, with brushed, guilloché, or skeletonized finishes. Tungsten is preferred when high density is needed in a small volume, because greater mass generates more torque with smaller oscillations. 22K gold, rich in copper, is chosen for aesthetic reasons and works well due to its high specific weight.
The micro-rotor represents a different constructive approach: the mass is made smaller and integrated within the plane of the movement, often decentered from the central axis. It gains thickness—even three or four tenths of a millimeter make a difference on an ultra-thin watch—but requires greater manufacturing precision and ball bearings to compensate for increased friction. Calibers like the Panerai P.4100 and Czapek SXH5 use it: in both cases, it frees up space for additional functions or to reduce the total case height.
The peripheral rotor is the third option: the mass runs along the perimeter of the movement, leaving the center completely open to view. The Breguet 581DPE is an example I know well. Constructively it is more complex than a central rotor, but less critical than a micro-rotor in terms of thickness constraints.
Historically, the first automatic rotor was the bumper, invented by John Harwood in 1923: it oscillated only 180° and was stopped by lateral springs. Less efficient, but Omega models from the 1940s and 1950s with that mechanism produce a distinctive ticking that collectors recognize by ear. In 1942 Felsa introduced the Bidynator, the first bidirectional 360° rotor, which is the direct ancestor of everything we use today.
Regarding rotor skeletonization: lightening does not mean weakening if only non-structural material is removed. It reduces inertia and can improve response to slow oscillations, but the real benefit depends on the rest of the gear train. Alternative systems like Ulysse Nardin's Grinder abandon the classical rotor and use a multi-arm frame to convert even microvibrations into energy: interesting from an engineering standpoint, less proven over the long term.