An electret is a ferroelectric material that has been permanently electrically charged or "polarized". An electret driver functions along the same electromechanical means as an electrostatic driver. Many players have a built-in electret microphone which allows recording. It is often used for powering electret microphones, which will not function without power. For example, they are used in electret microphones and in copy machines. Thermo-forming of layer systems from electret films led to thermally more stable ferroelectrets. An electret is the electrical equivalent of a permanent magnet. New ceramics that exhibit giant electret hysteresis have also been realized. Excess charge within an electret decays exponentially. Electret and electrostatic headphones are relatively uncommon. Electret materials are quite common in nature. Electret materials have found commercial and technical interest. Electret microphone bias is sometimes supplied on a separate conductor. It was in 1925 that the first piezoelectric polymer was discovered (Electret).