50 Electric and Magnetic Science 



<v 



the distance between the acting charges decreases, Aepinus 

 applied his theory to explain a phenomenon which had been 

 more or less indefinitely observed by many previous writers, and 

 specially studied a short time previously by John Canton* 

 (&. 1718, d. 1772) and by Wilckef 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} 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 did 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 accom- 

 plished 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. " May we not infer," he says, "from 



*Phil. Trans, xlviii (1753), p. 350. 



t Disputatio physica experimentalis de electricitatibus contrariis : Rostock, 1757. 



J In liis paper read to the Royal Society on Dec. 18th, 1755. 



J. Priestley, The History and Present State of Electricity, with Original 

 Experiments ; London, 1767: page 732. That electrical attraction follows the 

 law of the inverse square had been suspected -by Daniel Bernoulli in 1760: Cf. 

 Sochi's Experiments, Ada Helvetica, iv, p. 214. 



