The term electret was coined by Oliver Heaviside[1] for a (typically dielectric) material which has electrical charges of opposite sign at its extremities.[2] Some materials with electret properties were already known to science and had been studied since the early 1700s. One example is the electrophorus, a device consisting of a slab with electret properties and a separate metal plate. The electrophorus was originally invented by Johan Carl Wilcke in Sweden in 1762[3] and improved by Alessandro Volta in Italy in 1775.[4] The first documented case of production was by Mototarô Eguchi in 1925[5] who melting a suitable dielectric material such as a polymer or wax that contains polar molecules, and then allowing it to solidify in a powerful electric field. The polar molecules of the dielectric align themselves to the direction of the electric field, producing a dipole electret with a permanent polarization. Modern electrets are sometimes made by embedding excess charges into a highly insulating dielectric, e.g. using an electron beam, corona discharge, injection from an electron gun, electric breakdown across a gap, or a dielectric barrier.[6][7]
^Heaviside, Oliver (1894). Electrical Papers. Macmillan and Company. pp. 488–493.
