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 electrical 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 instituted 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 published the first of his seven memoirs, on the data of which the mathematical 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 throughout 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.
