Cavendish independently conceived a theory of electricity nearly akin to that of Aepinus ('Philosophical Transactions,) 1771). He also (1784) was perhaps the first to utilize the elec tric spark to produce the explosion of hydrogen and oxygen in the propef proportions to pro duce pure water. The same philosopher also discovered the inductive capacity of dielectrics (insulators) and as early as 1775 measured the specific inductive capacity for beeswax and other substances by comparison with an air condenser.
About 1784 C. A. Coulomb, after whom is named the electrical unit of quantity, devised the torsion balance, by means of which he dis covered what is known as Coulomb's law;— The force exerted between two small electrified bodies varies inversely as the square of the distance; not as Aepinus in his theory of elec tricity had assumed, merely inversely as the distance. According to the theory advanced by Cavendish "the particles attract and are at tracted inversely as some less power of the distance than the cube." With the discovery, by the experiments of Watson and others, that electricity could be transmitted to a distance, the idea of making practical use of this phenomenon began, about 1753, to engross the minds of ((inquisitive)) per sons, and to this end suggestions looking to the employment of electricity in the transmission of intelligence were made. The first of the meth ods devised for thispurpose was probably that due to Lesage (1774). This method consisted in the employment of 24 wires, insulated from one another and each of which had a pith ball connected to its distant end. Each wire repre sented a letter of the alphabet. To send a mes sage, a desired wire was charged momentarily with electricity from an electric machine, whereupon the pith ball connected to that wire would fly out; and in this way messages were transmitted. Other methods of telegraphing in which frictional electricity was employed were also tried, some of which are described in the article on the telegraph (q.v.).
Hitherto the only electricity known was that developed by friction or rubbing, which was therefore termed frictional electricity. We now come to the era of galvanic or voltaic electricity. The first mention of voltaic electricity, although not recognized as such at the time, was prob ably made by Sulzer in 1767 who on placing a small disc of zinc under his tongue and a small disc of copper over it, observed a pecul iar taste when the respective metals touched at their edges. Sulzer assumed that when the
metals came together they were set into vibra tion, this acting upon the nerves of the tongue, producing the effects noticed.
In 1790 Prof. Luigi Alvisio Galvani of Bologna on one occasion, while conducting ex periments on (animal electricity?) as he termed it, to which his attention had been turned by the twitching of a frog's legs in the presence of an electric machine, observed that the muscles of a frog which was suspended on an iron balus trade by a copper hook that passed through its dorsal column underwent lively convulsions without any extraneous cause; the electric ma chine being at this time absent. To account for this phenomenon Galvani assumed that elec tricity of opposite kinds existed in the nerves and muscles of the frog; the muscles and nerves constituting the charged coatings of a Leyden jar.
Galvani published the results of his discov eries, together with his hypothesis, which at once engrossed the attention of the physicists of that time; the most prominent of whom, Alexander Volta, professor of physics at Pavia, contended that the results observed by Galvani were due to the two metals, copper and iron, acting as telectromotors?" and that the muscles of the frog played the part of a conductor, completing the circuit.
This precipitated a long discussion between the adherents of the conflicting views; one set of adherents holding with Volta that the elec tric current was the result of an electromotive force of contact at the two metals; the other set adopting a modification of Galvani's view and asserting that the current was due to a chem ical affinity between the metals and the acids in the pile. Michael Faraday wrote in the preface to his