Electret

Electret, material that maintains electrical polarity after exposure to a powerful electric field. In such a material, one end has a slightly positive electric charge and the other end has a slightly negative charge, but the net charge of the material is zero. Electrets are widely used in all kinds of microphones, from telephone mouthpieces to hearing aids. They are also used in loudspeakers, some electrical meters, and keys or buttons on devices such as telephones and calculators.

Electrets are made from materials such as wax, plastic, or ceramics. The individual molecules of these materials each have a positively charged and negatively charged end. These polarized molecules are arranged randomly so that the material has no overall polarization. When the material is exposed to a strong electric field (about 1 million v/m, or about 300,000 v/ft) the molecules of the material rotate into permanent alignment and their orientation remains unchanged even after they are no longer subject to the electrical field. Some electrets are created by subjecting molten material to a strong electric field, then allowing the material to harden while still in the field.

The electret microphone of a telephone, the most familiar application of an electret, is made by solidifying a molten material. In this type of microphone, a diaphragm that vibrates as sound waves hit it is made of a plastic electret coated with a thin film of metal. The metal side of the diaphragm faces the mouthpiece and the plastic side faces a metal disk. The permanent charge of the electret causes an electric field between the diaphragm and the disk. As the diaphragm vibrates, the distance between it and the disk changes, affecting the intensity of the electric field. This produces an electric current through wires attached to the diaphragm and disk. This current can be sent through telephone lines and translated back into sound by the receiver of another telephone.