A crystal of quartz suitably cut and mounted will produce an electrical effect when mechanically deformed; conversely, it mechanically deforms when subjected to an electrical field. This is known as the 'piezo-electric effect'. When a suitably cut quartz crystal is connected in an electrical circuit, it can be made to vibrate at its natural frequency, and may be used to control the frequency of an electronic oscillator which, after suitable frequency division, will indicate time on a digital or analogue display.