Electrophorus




An electrophorus is a capacitive  generator used to produce electrostatic charge via the process of electrostatic induction.
The basic components of an electrophorus are a flat, plate-shaped insulator, an insulating handle and a metal disk. The insulator, usually called a “cake,” can be made from a variety of materials. Unlike the cake, the metal disk is a conductor. In the electrophorus, the disk acts as an electrode by allowing current to pass through to a nonmetallic medium. The metal disk of an electrophorus attaches to an insulating handle and the cake stands alone.
The dielectric plate is charged through the by rubbing it with fur or cloth. Let us suppose that  the dielectric gains negative charge by rubbing. Then, the metal plate is placed onto the dielectric plate. Due to electrostatic induction it develops two regions of charge — the positive charges in the plate are attracted to the side facing down toward the dielectric, charging it positively, while the negative charges are repelled to the side facing up, charging it negatively, with the plate remaining electrically neutral as a whole. Then, the side facing up is momentarily grounded (which can be done by touching it with a finger), draining off the negative charge. Finally, the metal plate, now carrying only positive sign of charge  is lifted.
Since the charge on the dielectric is not depleted in this process, the charge on the metal plate can be used for experiments, for example by touching it to metal conductors allowing the charge to drain away, and the uncharged metal plate can be placed back on the dielectric and the process repeated to get another charge. This can be repeated as often as desired, so in principle an unlimited amount of induced charge can be obtained from a single charge on the dielectric. In actual use the charge on the dielectric will eventually  leak off through the surface of the dielectric  or the atmosphere to recombine with opposite charges around to restore neutrality.