We learned that a digital clock is regulated by measuring how many times a quartz crystal oscillates per second—32,768 times. How does it count all those vibrations so quickly? Here’s how: the crystal is purposely cut with a laser to exactly the size and shape (the shape of a tuning fork) that will produce 32,768 oscillations in a second, then stop.* The electric circuit zaps the crystal with electricity, which makes the crystal vibrate until it returns to its original shape. When the vibrating stops, exactly one second has passed. The stopped vibrations trigger the circuit to move the second hand and give the crystal another zap.
Here’s how a tuning fork works: https://www.youtube.com/watch?v=hW-igtIn3A8
* “Because 32768Hz can be so conveniently divided to give a 1 second pulse, it is a very popular size for it to be cut to. Manufacturers can bang them out and be sure they will sell.” https://www.quora.com/Why-does-Quartz-vibrate-exactly-32768-2-15-times-per-second
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