Explainer No. 3
No battery and no motor: just a coiled spring, a chain of gears and a little wheel that swings back and forth. Here it is, enlarged and slowed down.
Point at a part to find it in the drawings. The winding and hand-setting parts are left out.
A long coiled steel ribbon. Winding stores energy in it; it gives that energy back as it unwinds.
The toothed drum the mainspring lives in. It turns slowly as the spring lets out.
Turns once an hour and carries the minute hand.
A step up in speed between the center wheel and the fourth wheel.
Turns once a minute, so it can carry the seconds hand.
Usually fifteen teeth, let go half a tooth at a time.
Rocks to and fro, blocking and releasing the escape wheel with its two jewels.
The timekeeper. It swings there and back three times a second here, a tick and a tock each time, and its roller jewel flicks the fork.
A fine spiral that keeps pulling the balance back toward the middle of its swing.
Hours, minutes and small seconds, driven by the train.
Synthetic ruby or sapphire bearings the pivots turn in. The pallet stones and the roller jewel are jewels too, riding on the fork and the balance.
The mainspring would happily unwind all at once. The escapement only lets the gear train move half a tooth at a time, each time the balance swings past the middle. The energy comes out in tiny equal pieces, and the balance sets the pace.
The tick is the escapement at work: the escape wheel stopping dead each time a tooth lands on a pallet jewel. Here that happens six times a second, and each one is a beat.
The seconds hand only moves when the escape wheel does, so it advances in small steps, one per beat. At 21,600 beats an hour that's six little steps every second, quick enough to look almost like a sweep.
The "jewels" are bearings and working faces made of synthetic ruby or sapphire. They're hard and smooth, so pivots and pallets wear more slowly. A typical fully jeweled watch that only tells the time has 17 of them.
Christiaan Huygens introduced the balance spring in 1675 (Robert Hooke disputed the credit), and pocket watches became far more accurate. Thomas Mudge invented the lever escapement in the 1750s; a descendant of it ticks in almost every mechanical watch made today.
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