1 8 Introduction 



overestimate the delicacy and ingenuity of his apparatus, the accuracy of 

 his observations, and the sound scientific method of his researches ; but it 

 is remarkable, that not one of his experiments coincides with any of those 

 made by Cavendish. The method by which Coulomb made direct measure- 

 ments of the electric force at different distances, and that by which he 

 compared the density of the surface-charge on different parts of conductors, 

 are entirely his own, and were not anticipated by Cavendish. On the 

 other hand, the very idea of the capacity of a conductor as a subject of 

 investigation is entirely due to Cavendish, and nothing equivalent to it 

 is to be found in the memoirs of Coulomb. 



The leading idea which distinguishes the electrical researches of 

 Cavendish from those of his predecessors and contemporaries, is the 

 introduction of the phrase "degree of electrification " with a clear scientific 

 definition, which shows that it is precisely equivalent to what we now 

 call potential. 



In his first published paper (1771), he begins at Art. 101 by giving a 

 precise sense to the terms "positively and negatively electrified," which 

 up to that time had been in common use, but were often confounded with 

 the terms "over and under charged," and in Art. 102 he defines what is 

 meant by the "degree of electrification." 



We find the same idea, however, in the much earlier draft of his theory 

 in the "Thoughts concerning Electricity," Art. 201, where the degree of 

 electrification is boldly, if somewhat prematurely, explained in a physical 

 sense, as the compression, or as we should now say, the pressure, of the 

 electric fluid. 



We can trace this leading idea through the whole course of the 

 electrical researches. 



He shows that when two charged conductors are connected by a wire 

 they must be electrified in the same degree, and he devotes the greater 

 part of his experimental work to the comparison of the charges of the 

 two bodies when equally electrified. 



He ascertained by a well-arranged series of experiments the ratios of 

 the charges of a great number of bodies to that of a sphere 12-1 inches in 

 diameter, and as he had already proved that the charges of similar bodies 

 are in the ratio of their linear dimenions, he expressed the charge of any given 

 body in terms of the diameter of the sphere, which, when equally electrified, 

 would have an equal charge, so that when in his private journals he speaks 

 of the charge of a body as being so many "globular inches," or more briefly, 

 so many "inches of electricity," he means that the capacity of the body 

 is equal to that of a sphere whose diameter is that number of inches. 



In the present state of electrical science, the capacity of a body is 

 defined as its charge when its potential is unity, and the capacity of a 

 sphere as thus defined is numerically equal to its radius. Hence, when 

 Cavendish says that a certain conductor contains n inches of electricity, 



