ELECTRET

material that retains its electric polarization after being subjected to a strong electric field. The positive charge within the material becomes permanently displaced in the direction of the field, and the negative charge becomes permanently displaced in the direction opposite to the field. One end of the electret is somewhat positive, and the other is somewhat negative, though the net charge remains zero. Electrets are prepared from certain waxes, plastics, and ceramics, the individual molecules of which are permanently polarized but are randomly arranged before being subjected to an electric field so that there is no overall polarization in the material. The strong electric field (approximately 1,000,000 volts per metre) rotates the polar molecules into an alignment that persists when the external field is removed. Sometimes electrets are made by allowing a molten material to solidify in a strong electric field. The behaviour of electrets in an electric field is analogous to that of permanent magnets in a magnetic field. An electret, for example, lines up in an electric field with its positive end pointing in the direction of the field. Electrets, discovered in 1925, have found applications in electrostatic microphones.