Herman Wilson, age 17, of Chester, Pa., for his question:

CAN YOU EXPLAIN THE PIEZOELECTRIC EFFECT?

The piezoelectric effect is the appearance of an electric potential across certain faces of a crystal when it is subjected to mechanical pressure. Conversely, when an electric field is applied on certain faces of the crystal, the crystal undergoes mechanical distortion.

A French scientist named Pierre Curie and his brother Jacques discovered the phenomenon in quartz and Rochelle salt in 1880 and named the effect piezoelectricity, which in Greek means "to press."

The piezoelectric effect occurs in several crystalline substances. The effect is explained by the displacement of ions in crystals that have a non symmetrical unit cell, the simplest polyhedron that makes up the crystal structure.

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.

Piezoelectric crystals are also used as resonators in electronic oscillators and high frequency amplifiers because the mechanical resonance frequency is stable and well defined.