electrophorus

electrophorus

[ih-lek-trof-er-uhs, ee-lek-]
electrophorus, device used to generate static electric charges. It has two parts: a nonconducting plate (e.g., of hard rubber) that is negatively charged and a metal plate with an insulated handle. A positive charge is induced on the metal plate by placing it on the charged plate and then grounding the metal plate momentarily. This positive charge can then be used for experimentation. See electrostatics.

An electrophorus is a capacitive generator used to produce electrostatic charge via the process of electrostatic induction. It was invented in 1764 by Swedish professor Johan Carl Wilcke, but Italian scientist Alessandro Volta improved and popularized the device in 1775, and is sometimes erroneously credited with its invention. The word electrophorus was coined by Volta from the Greek ήλεκτρον ('elektron'), ϕέρω ('phero'), meaning 'electricity bearer'.

The electrophorus consists of a dielectric plate (originally a 'cake' of resinous material like pitch or wax, but in modern versions plastic is used) and a metal plate with an insulating handle. First, the dielectric plate is charged through the triboelectric effect by rubbing it with fur or cloth. Then, the metal plate is placed onto the dielectric plate. The electrostatic field of the dielectric causes the charges in the metal plate to separate. The metal develops two regions of charge — the side facing the charged dielectric plate charges opposite to the charge of plate, while the side facing away from the dielectric charge attains the same sign of charge as the dielectric plate, with the metal plate remaining electrically neutral as a whole. Then, the side facing away from the dielectric plate is momentarily grounded (which can be done by touching it with a finger), draining off the alike charge. Finally, the metal plate, now carrying only one sign of charge, is lifted.

The charge on the plate can be discharged and the process can be repeated, replacing the plate on the dielectric and grounding the top to get a new charge on the plate. This can be repeated as often as desired without depleting the dielectric's charge, and in this way an unlimited amount of charge can be obtained from the device (although in actual use the charge on the dielectric will eventually leak away through the atmosphere). For this reason Volta called it elettroforo perpetuo (the perpetual electrophorus).

One of the largest examples of an electrophorus was built in 1777 by German scientist Georg Christoph Lichtenberg. It was 6 feet (2 m) in diameter, with the metal plate raised and lowered using a pulley system. It could reportedly produce 15 inch (38 cm) sparks. Lichtenberg used its discharges to create the strange treelike marks known as Lichtenberg figures.

Where does the charge come from?

It is sometimes asked, how can an unlimited amount of charge be gotten from the limited initial charge on the device? The answer is that the charge on the dielectric isn't consumed in the process. Its role is just to induce charge in the plate. Although the plate is set on the dielectric, it only makes contact with the surface in a few places, and little or no charge is transferred since charge can't move through the dielectric; in fact the electrophorus can function without the two parts touching.

Where does the energy for all this electricity come from? The energy to accumulate each charge comes from the work done in lifting the charged plate away from the dielectric surface, against the electrostatic force between them. The electrophorus is actually a manually operated electrostatic generator, using the same induction principle as electrostatic machines such as the Wimshurst machine and the Van de Graaf generator.

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Notes

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