438 POPULAR SCIENCE MONTHLY. 



completely these researches of Cavendish remained unknown to other 

 men of science is shown by the subsequent external history of electricity. 

 Cavendish's work as a member of the committee appointed by the 

 Royal Society to investigate protection from lightning shows him 

 cooperating with Franklin and others in an investigation on behalf of 

 the nation. But most of his work was a private matter and in elec- 

 trical science, in which he was by far the authority of his day, he 

 published only two papers, 'Of the Electrical Property of the Torpedo' 

 (1776) and 'An Attempt to explain some of the principal Phenomena 

 of Electricity by means of an Elastic Fluid' (1771-72). Yet he left 

 behind him some twenty packets of manuscript on mathematical and 

 experimental electricity, which were but little known till Maxwell 

 edited them in 1879, for they were only alluded to in his celebrated 

 paper on the Torpedo. They anticipated, however, many of the facts 

 subsequently made known by Coulomb and other celebrated physicists, 

 and contained some of the results of experiments of a refined kind in- 

 stituted at a much later day. 



Cavendish proved the law of inverse squares for electric charges 

 not by actually measuring the forces as in the case of gravitational 

 attraction, but by showing that the entire charge resides on the surface 

 of a charged body and that there is no charge at all communicated to a 

 sphere within a sphere when electrically connected and a positive 

 charge is given to the outer one. He then established the theorem 

 that the force must vary inversely as the second power of the distance 

 between charges in order to explain this result of experiment, showing 

 that if it varied according to any higher power, the inner globe would 

 receive a part of the positive charge, if according to any lower power, 

 the inner globe would be negatively charged. 



These experiments on the law of inverse squares were performed in 

 December, 1772, and in fact all of his work in electrostatics was 

 completed before 1774, while it was not till 1785 that Coulomb pub- 

 lished the first of his seven memoirs, on the data of which the mathe- 

 matical theory of electricity as we now know it was founded by Poisson ; 

 and as Cavendish never published his at all, it is plain that each 

 worked in ignorance of the other's results. The method of each was 

 distinctly his own. Coulomb made direct measurements of the electric 

 force at different distances and compared the density of the surface 

 charge on different parts of conductors. On the other hand, the very 

 idea of the capacity of a conductor as a subject for investigation is due 

 entirely to Cavendish, and nothing equivalent to it occurs in the 

 memoirs of Coulomb. The method that Cavendish adhered to through- 

 out his experimental work was the comparison of capacities, and the 

 formation of a graduated series of condensers, such as is now regarded 

 as the most important apparatus in electrostatic measurements. 



