An electrophorus is a form of primitive static electrical charge battery or generator that was initially invented in 1762 by a Swedish physicist, Johan Wilcke, yet the function of the device is basic enough that it can be made from a variety of common materials. The noted political figure and inventor in colonial America, Benjamin Franklin, popularized the idea by making one out of wood, sulfur, wax, and pewter metal. A conventional electrophorus consists of an insulating plate, usually made of resin or plastic of some type, upon which a metal conducting plate is placed. In the center of the metal plate is affixed another insulating medium such as a plastic cup that is used as a handle to prevent premature discharge of the electrophorus. The static electric charges can be drawn off of the device by physical touch to the metal plate, either momentarily charging light bulbs or to perform other experiments.
The basic principle behind the operation of an electrophorus is that of the triboelectric effect, otherwise known as stored static electricity. The insulating plate is first charged with a static electrical charge before an electrophorus is assembled by rubbing it against other materials such as wool that will induce an electrical charge in the insulating plate through the process of electrostatic induction. Once the insulating plate and conducting plate are placed together, the static charge in the insulator separates out positive and negative charges in the metal conductor. Positive charges in the metal are drawn down towards the insulating plate, and negative charges are repelled up.
The induced negative charge in the upper surface of the metal plate can then be discharged by making physical contact with the metal, completing a circuit through the human body into the ground. The charge is often strong enough that a harmless spark will leap from the metal to one's finger before contact, or the leads of a small light bulb can be placed between the two surfaces to temporarily light it as the charge dissipates. Large versions of electrophorus will also light up a tube-shaped fluorescent light bulb momentarily if one end is held in a hand and the other is held close to the charged metal plate.
One of the unique aspects of an electrophorus that makes it a popular demonstration of electrical principles is that the insulating plate acts similarly to more sophisticated capacitors or electrical storage units in common electronic circuits. Unlike typical capacitors, however, the insulating plate holds a charge that is not depleted by the process of static discharge in the metal plate. After the metal plate's polarized discharge of energy has occurred, if the metal plate is removed from the vicinity of the insulating plate, then placed atop it again, the charge separation in the metal will repeat as the insulator still holds its charge.
This process appears to demonstrate the idea of free energy coming from nowhere, as the static charge is continually renewed and discharged no matter how many times the metal plate is removed and then placed back atop the insulating plate. In reality, energy is conserved because the work of physically moving the metal plate away and then replacing it introduces potential energy into the electrophorus system, which is then converted to kinetic energy when a discharge or spark takes place. Large versions of electrophorus are known as Van de Graaf generators, which are capable of producing electrostatic voltages in the range of up to 2,000,000 volts, such as the one created by the US physicist Dr. Robert Van de Graaf himself in the early 1930s.