What is Electret?
It is a dielectric (insulator) that has semi-permanent electric charge. Dr. Heaviside, the English physicist, named this substance Electret based on the permanent magnet. In the 1920s, Dr. Mototaro Eguchi, who was in the faculty of the Japanese Naval College at that time, successfully fabricated the world’s first electret using a thermal charging method.

When an electrode is opposed to electret, induced charges are generated. When the electrode slides while vibrating, the induced charges are transferred, causing an alternating current in the external circuit.