logical system clock to physical system clock.

The basic principle behind all watches and clocks is a device that oscillates at a fixed frequency. Early mechanical watches and clocks used a pendulum or springs with some form of regulator to keep the frequency fixed. Time is simply measured by counting these oscillations. A simple device might oscillate at 1Hz (once per second), therefore 1 second would elapse for each oscillation.

Quartz watches began to appear in the early 1970s. They use a crystal of quartz (silicon dioxide) shaped like a small bar. This is a 'piezoelectric' material - when bent or compressed it generates a small electric field (and vice versa).

The crystal is formed to have a natural oscillation at around 32,000Hz. These oscillations generate small electrical signals which are 'divided down' by the circuit within the watch to the required frequency (usually seconds) and translated into pulses which are sent to the watch display or a motor to move the seconds hand.

The advantage of quartz watches is their simplicity and accuracy - crystals maintain their frequency over broad operating conditions and are cheap to make.

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.