the distance between the acting charges decreases, Aepinus applied his theory to explain a phenomenon which bad been more or less indefinitely observed by many previous writers, and specially studied a short time previously by John Canton[1] (b. 1718, d. 1772) and by Wilcket[2]—namely, that if a conductor is brought into the neighbourhood of an excited body without actually touching it, the remoter portion of the conductor acquires an electric charge of the same kind as that of the excited body, while the nearer portion acquires a charge of the opposite kind. This effect, which is known as the induction of electric charges, had been explained by Canton himself and by Franklin[3] in terms of the theory of electric effluvia. Aepinus showed that it followed naturally from the theory of action at a distance, by taking into account the mobility of the electric fluid in conductors; and by discussing different cases, so far as was possible with the means at his command, he laid the foundations of the mathematical theory of electrostatics.
Aepinus (lid not succeed in determining the law according to which the force between two electric charges varies with the distance between them; and the honour of having first accomplished this belongs to Joseph Priestley (b. 1733, d. 1804), the discoverer of oxygen. Priestley, who was a friend of Franklin's, had been informed by the latter that he had found cork balls to be wholly unaffected by the electricity of a metal cup within which they were held; and Franklin desired priestley to repeat and ascertain the fact. Accordingly, on December 21st, 1766, Priestley instituted experiments, which showed that, when a hollow metallic vessel is electrified, there is no charge on the inner surface (except near the opening), and no electric force in the air inside. From this he at once drew the correct conclusion, which was published in 1767.[4] "May we not infer," he says, "from
- ↑ Phil. Trans. xlviii (1763), p. 350.
- ↑ Disputatio physica experimentalis de electricitatibus contrariis: Rostock, 1757.
- ↑ In his paper read to the Royal Society on Dee. 1811, 1766.
- ↑ Priestley, The History and Present State of Electricity, with Original Experimens; London, 1767: page 732. That electrical attraction follows the law of the inverse square had been suspected by Daniel Bernoulli in 1700: Cr. Socin's Experiments, Acta Helvetica, iv, p. 214.
