The watch uses a tiny piece of quartz crystal which is the main ingredient of sand “silicon-dioxide” , Quartz crystals are piezoelectric, which means that they generate an electrical charge when mechanical pressure is applied to them. They also vibrate if an electrical charge is applied to them. The frequency of this vibration is a function of the cut and shape of the crystal. Quartz crystals can be cut at a consistent size and shape to vibrate at thousands of times per second(around 32,000Hz), making them extremely stable resonators for keeping very accurate time.
If you have read my previous posts, you might be knowing what exactly “Piezoelectricity” is. If you missed it here’s the link Piezoelectricity.
That quartz crystal serves as the oscillator. The battery sends electricity to the quartz crystal through an electronic circuit. The circuit counts the vibrations and generates regular electric pulses of one per second. Now the motor comes into picture, the electric pulses when reaches the motor, the motor turns the seconds hand and the clock works,
Higher accuracy devices generally use a material with even higher frequencies (again the frequency must be as stable as possible). Atomic clocks count the oscillations between the nucleus and the electrons in an atom (typically cesium) which oscillate at around 9 billion Hz.