The oscillating mass is the component that transforms wrist movement into mechanical energy, winding the mainspring of an automatic movement. It's not a recent invention: the principle dates back to John Harwood's 1924 patent, where the rotor moved through an arc of approximately 270° striking bumper springs on the sides, a system later called bumper. Rolex was among the first to switch to full 360° rotation, which became the standard for almost the entire twentieth century.
The material of the oscillating mass directly affects charging efficiency. Tungsten is widely used today because it has extremely high density: at equal volume, it weighs more and generates more torque for each degree of rotation. Gold — particularly 21-carat gold, or 5N — is a more traditional choice, often associated with high-end movements where the sapphire caseback makes the rotor visible as an aesthetic element. Platinum 950, used for example in the Tonda PF, takes the concept even further: the densest material available, finished with the same care as the dial.
The most important distinction, from a technical standpoint, is between central rotor and microrotor. The central rotor rotates around the movement's axis and covers almost the entire caliber surface; the microrotor, introduced by Büren in the 1950s and later adopted by many manufactures, has a reduced diameter and is integrated at the level of the plate or bridges, allowing thicknesses on the order of 3-4 mm in the finished movement. Chopard's L.U.C 96.22-L caliber achieves 6 mm total thickness for the entire watch precisely because of this solution. The microrotor comes at a cost in terms of power reserve: the mass is smaller, and thinner movements often require shorter mainsprings.
There is also the peripheral rotor, where the oscillating mass runs on the outer edge of the movement rather than at the center: Perrelet was the historical pioneer, and some recent implementations make the rotor visible from the dial side rather than the caseback. It's a mechanically complex solution — it requires a peripheral gear ring and a pinion — but it completely frees up the center of the movement for decorations or complications.
The rotor's attachment to the bearing is a detail rarely discussed but one that matters in the workshop: crimping is the cleanest system but makes the rotor non-removable without specific equipment; locking plates allow disassembly but add complexity; the nut is aesthetically visible. Some recent solutions use central threading that screws directly onto the bearing crown, combining ease of disassembly and low profile.