The piezoelectric effect is a reversible relationship between mechanical stress and electrostatic potential exhibited by certain crystals, including quartz, and crystals containing barium titanate and tourmaline. Piezoelectricity, and the effect that gives rise to it, was discovered in 1880 during investigation of pyroelectric crystals (these are also asymmetric and get oppositely charged faces when heated).
When pressure is applied to a piezoelectric crystal, positive and negative electric charges appear on opposite crystal faces. Replacing the pressure by tension changes the sign of the charges. If, instead, an electric potential is applied across the crystal, its length changes; this effect is linear.
A piezoelectric crystal placed in an alternating electric circuit will alternately expand and contract (i.e., mechanical deformation of the crystal results). Resonance occurs in the circuit when its frequency matches the natural vibration frequency of the crystal, this effect being applied in frequency controllers. This useful way of coupling electrical and mechanical effects is used in a large variety of electromechanical transducers, such as microphones, phonographic pick-ups, pressure sensors, and accelerometers.