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.