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In a mechanical watch, the watch’s gears are turned by a spiral spring called a mainspring. In a manual watch, energy is stored in the mainspring by turning a knob, the crown on the side of the watch. Then the energy from the mainspring powers the watch movement until it runs down, requiring the spring to be wound again.
A self-winding watch movement has a mechanism that winds the mainspring using the natural motions of the wearer’s body. The watch contains an oscillating weight that turns on a pivot. The normal movements of the watch in the user’s pocket (for a pocket watch) or on the user’s arm (for a wristwatch) cause the rotor to pivot on its staff, which is attached to a ratcheted winding mechanism. The motion of the watch is thereby translated into circular motion of the weight which, through a series of reverser and reducing gears, eventually winds the mainspring. There are many different designs for modern self-winding mechanisms. Some designs allow the winding of the watch to take place while the weight swings in only one direction while other, more advanced, mechanisms have two ratchets and wind the mainspring during both clockwise and anti-clockwise weight motions.
The fully wound mainspring in a typical watch can store enough energy reserve for roughly two days, allowing the watch to keep running through the night while stationary. In many cases automatic wristwatches can also be wound manually by turning the crown, so the watch can be kept running when not worn, and in case the wearer’s wrist motions are not sufficient to keep it wound automatically.