A solid dielectric with a quasipermanent electric moment. Electrets may be classified as real-charge electrets and dipolar-charge electrets. Real-charge electrets of present importance are either solid dielectrics with charges of one polarity at or near one side of the dielectric and charges of opposite polarity at or near the other side, or cellular dielectrics with charges of opposite polarity on the two sides of the voids. The dipolar-charge electrets are dielectrics with aligned dipolar charges. Some dielectrics are capable of storing both real and dipolar charges. Examples of charge arrangements in electrets are shown in the illustration.