Interactive mechanical timekeeping
An automatic watch must capture irregular wrist motion, store it, release it in controlled packets, count those packets, and display the count.
Follow the causal chainThe oscillator sets the rhythm; the escapement maintains and counts it; the train carries energy and turns the hands.
Energy capture
The rotor is an eccentric mass. As it swings, reversing gears rectify motion into winding. The mainspring stores work as elastic deformation inside the barrel.
Most automatic movements use a slipping bridle so a fully wound spring can slide against the barrel wall rather than being damaged.
Transmission
The going train uses gear ratios to let a slow, strong barrel drive faster wheels. Each meshing pair reverses direction.
Escapement
The pallet alternately locks the escape wheel and lets one tooth pass. During the exchange it also returns a small impulse to the balance.
Oscillator
Greater hairspring stiffness k raises frequency; greater balance inertia I lowers it. The escapement supplies energy but does not create the natural period.
Beat rate
28,800 vph means eight beats per second and four complete balance oscillations per second. A beat is one half-oscillation.
Accuracy