Micro-Rotor

Definition

A small winding rotor set into the plane of an automatic movement rather than turning above it, so that the movement stays as thin as a hand-wound one. It is usually made of a heavy metal, gold or platinum, to recover the mass it loses in size. Patek Philippe's 240, introduced in 1977 with a 22-carat gold micro-rotor recessed into the mainplate, is the reference example and is 2.53 mm thick; its world-time version, the 240 HU, shows the type's advantage, since even the extra module keeps the movement under 4 mm. The trade is winding efficiency: a small rotor gathers less energy, which is why the 240 runs a slower 21,600 vph balance that asks less of it.