It’s the hairspring that made watches—which are just little clocks—possible. You wind up the hairspring and as it uncoils it releases energy to power the watch. Since the hairspring is small, watchmakers could miniaturize the balance wheel and gears, too.
But if you really want precise timekeeping, a watch’s design must have as few moving parts as possible. Watches were mechanical. Mechanical or analogue machines (a clock or steam engine or internal combustion engine) need constantly to be fussed with: you have to oil the gears; or correct for changes in temperature or humidity; friction slows down the machinery; you have to wind it or feed it fuel…if you could just get rid of those moving parts, you’d have a more reliable watch.
The switch away from analogue didn’t happen all at once. When batteries became small enough, the first electrically-powered watch showed up in 1957. It was battery-operated, but still had mechanical moving parts, like gears and a balance wheel. It was made by the Hamilton Watch Company of Lancaster, Pennsylvania.
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