Piezoelectric Effect When pressure is applied to one pair of opposite faces of crystal like quartz, tourmaline, Rochelle salt, etc., cut with their faces perpendicular to its optic axis, equal and opposite charges appear across its other faces as shown in figure. This phenomenon is known as piezoelectric effect. The frequency of the developed emf is equal to the frequency of dynamic pressure. The sign of the charge gets reversed if the crystal is subjected to tension instead of pressure. The electricity produced by means of piezoelectric effect is called piezoelectricity . The material which can undergo piezoelectric effect is called piezoelectric material or crystal. Note: The type of crystal used for ultrasonic production is generally the X-cut crystal. An X-cut crystal is piezoelectric crystal cut in a direction perpendicular to its X-axes(Electrical axes). The cross-section of a natural quartz crystal is hexagonal. The lines joining the midpoint of the op...