Definitions:

 
 

 direct piezoelectric effect :

appearance of an electric potential across certain faces of a crystal when it is subjected to mechanical pressure.

 

 reverse piezoelectric effect :

Conversely, when an electric field is applied on certain faces of the crystal, the crystal undergoes mechanical distortion. Pierre Curie and his brother Jacques (1855-1941) discovered the phenomenon in quartz and Rochelle salt in 1880 and named the effect piezoelectricity (from Gr. piezein, "to press").

   
   Because of their capacity to convert mechanical deformation into electric voltages, and electric voltages into mechanical motion, piezoelectric crystals are used in such devices as the transducer, record-playing pickup elements, and the microphone . They are also used as resonators in electronic oscillators and high-frequency amplifiers, because the mechanical resonance frequency of adequately cut crystals is stable and well defined.
 Quartz crystal : piezoelectric material