Contact electrification is a phrase that describes a phenomenon whereby surfaces become electrically charged, via a number of possible mechanisms, when two or more objects come within close proximity of one another. When two objects are "touched" together, sometimes the objects become spontaneously charged. One object may develop a net negative charge, while the other develops an equal and opposite positive charge. This effect may be caused by various physical processes – triboelectricity, the Volta effect, differing work functions of metals, and others which are collective referred to as contact electrification. 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, and thermocouples. Contact between materials is responsible for such modern electrical technology as semiconductor junction devices including radio detector diodes, photocells, LEDs, and thermoelectric cells. History 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. The Volta effect (described below) corresponds to a weak electric potential difference developed by the contact of different metals. Nowadays, this is often known as a contact potential difference. This effect was first discovered by Alessandro Volta, and can be measured using a capacitance electroscope comprising different metals. However, this effect does not, by itself, account for the action of electric batteries. A number of high voltage dry piles were invented between the early 19th century and the 1830s in an attempt to determine the answer to this question, and specifically to support Volta’s hypothesis of contact tension. The Oxford Electric Bell is one example. Francis Ronalds in 1814 was one of the first to realise that dry piles also worked through chemical reaction rather than metal to metal contact, even though corrosion was not visible due to the very small currents