Balance wheel with hair spring
A watch is a small sturdy machine for telling time, operated by a coiled-spring motor and controlled by the oscillations of another fine spring-the hairspring-as it pulls and is pushed by the balance wheel. The coiled mainspring unwinds, turning the barrel in which it is house, once in 8 hours. The barrel is geared through a transmission train of gears, each of which, because of the ratios between gears, turns faster than the gear which precedes it. The center wheel, turned by the mainspring barrel, revolves on in one hour, This wheel turns the third wheel once in 7 ½ minutes. A simple calculation will show that both sets of wheels have a gear ratio of 8:1. The ratio between the third and fourth wheel differs. It is 7½:1, turning the fourth wheel once in one minute. The seconds hand fits on the shaft of this fourth wheel.
The fourth wheel is geared to the escape wheel with its boot-shaped teeth. The escape wheel turns once in 6 seconds....
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