Complication guide
Automatic winding: how the complication works
An automatic, or self-winding, movement winds its own mainspring from the motion of the wearer's wrist through a pivoting rotor.
In an automatic movement a semicircular weight, the rotor, is free to pivot on a bearing at the centre of the movement. As the wrist moves, the rotor swings and its motion is geared down to wind the mainspring. A reversing mechanism captures the swing in both directions so the watch winds whichever way the rotor turns.
The rotor and its winding bridge add height above the base gear train, so an automatic caliber is taller than the equivalent hand-wound movement. That extra height is one of the dimensions that has to match when looking for a drop-in donor: an automatic and a manual movement of the same diameter are not interchangeable in a case cut for one or the other.