How are quartz crystals used in watches?
Put a voltage across a quartz crystal and it changes size very very slightly, when the voltage is released, the crystal drops back to its original size and a current is generated … which generates a voltage … which causes the pattern to repeat. It soon runs down – but if you arrange to give it a kick at the right point in the cycle to keep it going (like a children’s swing) you get a crystal oscillator which produces a very precise frequency. Circuits called “dividers” usually divide the frequency down to the one-a-second frequency needed to operate a watch.
