Volta effect

Contact electrification was an erroneous scientific theory from the Enlightenment that attempted to account for all the sources of electric charge known at the time. It has been superseded by more modern notions. In the late 18th century, scientists developed sensitive instruments for detecting 'electrification', otherwise known as electrostatic charge imbalance. The phenomenon of electrification by contact, or contact tension, was quickly discovered. When two objects were touched together, sometimes the objects became spontaneously charged. One object developed a net negative charge, while the other developed an equal and opposite positive charge. Then it was discovered that 'piles' of dissimilar metal disks separated by acid-soaked cloth, Voltaic piles, could also produce charge differences. Although it was later found that these effects were caused by different physical processes - triboelectricity, the Volta effect, differing work functions of metals, and others - at the time they were all thought to be caused by a common 'contact electrification' process.

The contact electrification phenomenon allowed the construction of so-called 'frictional' electrostatic generators such as Ramsden's or Winter's machines, but it also led directly to the development of useful devices such as batteries, fuel cells, electroplating, thermocouples. Contact between materials is responsible for such modern electrical technology as semiconductor junction devices including radio detector diodes, photocells, LEDs, and thermoelectric cells.

The theory held that static electricity was generated by means of contact between dissimilar materials, and was in close agreement with the principles of static electricity as then understood. It was eventually replaced by the current theory of electrochemistry, namely, that electricity is generated by the action of chemistry and the exchange of electrons between atoms making up the battery. An important fact leading to the rejection of the theory of contact tension was the observation that corrosion, that is, the chemical degradation of the battery, seemed unavoidable with its use, and that the more electricity was drawn from the battery, the faster the corrosion proceeded.

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