The generation of a potential difference across the opposite faces of certain nonconducting crystals (piezoelectric crystals) as a result of the application of mechanical stress between these faces. The electric polarization (see dielectric) produced is proportional to the stress and the direction of the polarization reverses if the stress changes from compression to tension. The reverse piezoelectric effect is the opposite phenomenon: if the opposite faces of a piezoelectric crystal are subjected to a potential difference, the crystal changes its shape. Rochelle salt and quartz are the most frequently used piezoelectric materials. While Rochelle salt produces the greater polarization for a given stress, quartz is more widely used as its crystals have greater strength and are stable at temperatures in excess of 100°C.
If a quartz plate is subjected to an alternating electric field, the reverse piezoelectric effect causes it to expand and contract at the field frequency. If this field frequency is made to coincide with the natural elastic frequency of the crystal, the plate resonates; the direct piezoelectric effect then augments the applied electric field. This is the basis of the crystal oscillator and the quartz clock. See also crystal microphone; crystal pick-up.