An electrophorus is a basic static electric charge generator consisting of an insulating plate and a conductive metal plate. The triboelectric effect charges the insulating plate, inducing a charge in the metal plate. The device can be used to extract static electrical charges and demonstrate electrical principles. The insulating plate acts as a capacitor, holding a charge that is not depleted by the discharge process. Larger versions are known as Van de Graaf generators, capable of producing electrostatic voltages up to 2,000,000 volts.
An electrophorus is a form of battery or primitive static electric charge generator that was initially invented in 1762 by a Swedish physicist, Johan Wilcke, but the function of the device is basic enough that it could be made from a variety of common materials. The well-known politician and inventor in colonial America, Benjamin Franklin popularized the idea by making one out of wood, sulphur, wax and pewter metal. A conventional electrophorus consists of an insulating plate, usually resin or plastic of some type, upon which a conductive metal plate is placed. In the middle of the metal plate is attached another insulating medium such as a plastic cup which is used as a handle to prevent premature discharge of the electrophorus. Static electrical charges can be extracted from the device by physically touching the metal plate, momentarily charging light bulbs, or performing other experiments.
The 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 electric charge before an electrophore is assembled by rubbing it against other materials such as wool which will induce an electric charge in the insulating plate through the process of electrostatic induction. Once the insulating plate and the conductive plate have been put together, the static charge in the insulator separates the positive and negative charges in the metal conductor. The positive charges in the metal are drawn towards the insulating plate and the negative charges are repelled.
The negative charge induced in the top 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 the finger before contact, or the leads of a small light bulb can be placed between the two surfaces to temporarily ignite it as the charge dissipates. Large versions of the electrophorus will also momentarily turn on a tube-shaped fluorescent bulb if one end is held in one hand and the other is held near 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 similar to more sophisticated capacitors or electrical storage units in common electronic circuits. Unlike typical capacitors, however, the insulating plate contains a charge that is not depleted by the static discharge process in the metal plate. After the polarized discharge of metal plate energy has occurred, if the metal plate is removed from the vicinity of the insulator plate, then placed back on top of it, the charge separation in the metal will repeat while the insulator still holds its charge.
This process appears to demonstrate the idea of free energy coming out of nowhere, as static charge is continually renewed and discharged no matter how many times the metal plate is removed and then put back on top of the insulating plate. In reality, energy is conserved because the work of physically pulling the metal plate apart and then replacing it introduces potential energy into the electrophorus system, which is then converted into kinetic energy when a discharge or spark occurs. 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 that created by US physicist Dr. Robert Van de Graaf himself in the early 1930s.
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