A groundbreaking instrument, the electrophorus can be used to produce a considerable amount of electric charge over and over again. Swedish physicist Johannes Wilcke invented the device in the 1760s. Then in the following decade Alessandro Volta improved its design and coined the name by which it is now known. Because it provided a renewable source of charge, the electrophorus quickly became popular in scientific laboratories in the 18th century. Scientists who often referred to it as the “perpetual electrophorus” could use the charge to carry out experiments or to perform electrostatic demonstrations.
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. In the days of Wilcke and Volta, researchers typically formed the cake out of shellac and resin or a kind of wax spread over glass, but any good insulator can be used. 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.
Two basic processes — triboelectrification and induction — underlie the functioning of the electrophorus. Triboelectrification involves rubbing one material with another in order to bring about a separation of charge. For instance, wool, silk or fur rubbed over the insulating plate of an electrophorus produces a negative charge along the surface of the plate. This is because the plate possesses a greater affinity for electrons than wool, silk and fur. Therefore, electrons from those materials are acquired by the plate and distribute along its surface. Triboelectrification works better with certain materials than with others. Also some materials tend to acquire a positive charge when rubbed and others a negative charge. A classification system called the triboelectric series gives details about the charging capacities of common materials.
When the plate, or cake, has been charged, the conductive metal disk is placed upon it. The insulating handle allows manipulation of the electrophorus without causing the device to discharge. Once the disk is near the cake, a charge is produced in the disk via induction. If the cake holds a negative charge, the electrons within the metal disk are repelled, which causes them to move away from the surface facing the cake to the surface facing up. Then, if the metal disk is touched, or “grounded,” the electrons it holds on the top surface flow out of the material to the ground. What remains is an overall positively charged disk. Pulling the disk away from the cake causes the positive charge, and any remaining negative charge, to distribute evenly across the disk. The induction does not cause a corresponding loss or rearrangement of charge in the cake because it’s an insulator, which is why it can be used repeatedly after it has been initially charged via friction.