Oscillating quartz crystal, photographed removed from its capsule packaging. Quartz crystals vibrate at a defined frequency when exposed to an applied alternating electric field. Such crystals are used as timers in quartz watches. The phenomenon of the interraction of electrical and mechanical variables in a medium is known as the piezoelectric effect. Conversely, the compression of a quartz crystal may generate an electrostatic voltage across the crystal. This principle has been applied in the ignition system for some cigarette lighters.