(adapted from http://www.engr.uky.edu/
~gedney/courses/ee468/
expmnt/ephorus.html.)

The operation of the electrophorus proceeds as follows: Initially, the top of the insulating styrene sheet is briskly rubbed with a piece of cloth (preferable wool, silk, or fur). This will create a negative static charge on the surface of the styrene .

The metal disk with an insulating handle is then placed on top of the charged styrene sheet. Positive charges are attracted at the bottom surface of the metal disk by Coulomb Force, and negative charges are repelled to the top surface of the metal disk polarizing the disk. Note that the charges in the metal disk are "displaced" by the Coulomb force. The top surface of the metal disk is then grounded by either touching it with a finger or using a direct connection to ground with a conductor. The negative charges will be neutralized while the positive charge remains at the bottom of the metal disk.

By holding the insulating handle, the metal disk is then removed from the top of the styrene. Once it is removed and is sufficiently far from the styrene sheet, the positive charge will distribute itself about the metal disk until an equilibrium is reached. The charged disk is then touched to the electroscope. This causes the deflection arm to repel from its center.

The above procedure is then repeated, but without rubbing the insulator with the cloth a priori. Before repeating these steps, the electroscope and the metal disk are grounded such that they are charge neutral. Then, the disk is placed on top of the styrene, the top is grounded, the disk is removed, and then the disk is touched to the electroscope. It is observed that the electroscope is deflected the same distance as in the previous case. In fact, this can be repeated again any number of times, and the amount of charge induced on the metal disk remains the same. The reason for this is that the charge established on the top of the styrene sheet by the process of triboelectrification is bound to the surface of the styrene. This charge is also stationary in that it cannot be redistributed on the styrene, nor can it conduct onto the the metal sheet. On the other hand, the charge in the metal disk is free to redistribute under the influence of Coulomb forces. When the disk is placed near the negative bound charge, the electrons are repelled by the negative bound charge leaving a net negative charge on the top surface. When grounded, electrons are actually conducted to ground to neutralize the top of the metal plate. This renders a net positive charge on the plate. Thus, it is a combination of the stationary charge on the styrene, free charge movement in the conductor, and ground supplying free charge that renders the electrophorus as a perpetually charged device.

See also E143