When crystals of certain materials are subjected to a mechanical stress, they generate electromotive force. If these crystals are subjected to an alternating electrical stress, they vibrate. The relationship between the mechanical and electrical properties of the crystal is the piezoelectric effect.
Quartz, rochelle salts, and tourmaline all exhibit the piezoelectric effect. Quartz crystals are used to control the frequency out-put of transmitters or any other equipment where exact frequency control is required. Rochelle salts are used in microphones; tourmaline may be used in pressure gauges.
