It was not till 1785 that the first of the seven electrical memoirs of M. Coulomb was published. The experiments recorded in these memoirs furnished the data on which the mathematical theory of electricity, as we now have it, was actually founded by Poisson, and it is impossible to 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 measurements 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 dimensions, 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, we may express his result in modern language by saying that its electric capacity is in inches.

In his early experiments he seems to have endeavoured to obtain a number of conductors as different as possible in form, of which the capacities should be nearly equal. Thus we find him comparing a pasteboard circle of 194 inches in diameter with his globe of 121 inches in diameter, but finding the charge. of the circle greater than that of the globe, he ever after uses a circle of tin plate, 18.5 inches in diameter, the capacity of which he found more nearly equal to that of the globe.

In like manner the first wire that he used was 96 inches long and 0·185 diameter, but afterwards he always used a wire of the same diameter, but 72 inches long, the capacity of which was more nearly equal to that of the globe.

He also provided himself with a set of glass plates coated with circles of tin-foil on both sides. These plates formed three sets of three of equal capacity, the capacities of the three sets being as 1, 3 and 9, with a tenth coated plate whose capacity was as 27.

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Besides these he had "double" plates of very small capacity made of two plates of glass stuck together, and also other plates of wax and rosin, the inductive capacity of these substances being, as he had already found, less than that of glass; and jars of larger capacity, ranging up to his great battery of 49 jars, whose capacity was 321000 "inches of electricity." In estimating the capacity of his battery, he used the method of repeated touching with a body of small capacity. (Arts. 412, 441, 582.) This method is the same as that used by MM. Weber and Kohlrausch in their classical investigation of the ratio of the electric units*.

Thus the method of experimental research which Cavendish adhered to was the comparison of capacities, and the formation of a graduated series of condensers, such as is now recognised as the most important apparatus in electrostatic measurements.

We have next to consider the steps by which he established the accuracy of his theory, and the discoveries he made respecting the electrical properties of different substances.

The

Cavendish himself, in his description of his experiments, has shown us the order in which he wishes us to consider them. first experiment† is that of the globe within two hemispheres, from which he proves that the electric force varies inversely as the square of the distance, or at least cannot differ from that ratio by more than a fiftieth part. The degree of accuracy of all the experiments was limited by the sensitiveness of the pith ball electrometer which he used. Bennett's gold leaf electrometer, which is much more sensitive, was not introduced till 1787, but in repeating the experiment we can now use Thomson's Quadrant electrometer, and thereby detect a deviation from the law of the inverse square not exceeding one in 72000. See Note 19.

The second experiment, Art. 235, is a repetition of the first with bodies of different shape.

The third experiment, Art. 265, shows that in comparing the charges of bodies, the place where the connecting wire touches the

M.

* Elektrodynamische Maasbestimmungen, Abh. iv. p. 235.

+ Arts, 217 to 235.

body, and the form of the connecting wire itself, are matters of indifference.

The fourth experiment, Art. 269, shows that the charges of bodies of the same shape and size, but of different substances, are equal.

The fifth, Art. 273, compares the charge of a large circle with that of two of half the diameter. According to the theory the charge of the large circle should be equal to that of the two small ones if they are at a great distance from each other, and equal to twice that of the small ones if they are close together. Cavendish tried them at three different distances and compared the results with his calculations.

The sixth experiment, Art. 279, compares one long wire with two of half the length and half the diameter, placed at different distances.

The seventh, Art. 281, compares the charges of a globe, a circle, a square, an oblong and three different cylinders, and the eighth, Art. 288, shows that the charge of the middle plate of three parallel plates is small compared with that of the two outer ones.

Cavendish next describes his experiments for comparison of the charges of coated plates of glass and other substances, but begins by examining the sources of error in measurements of this kind.

The first of these which he investigates is the spreading of electricity on the surface of the plates beyond the coatings of tin` foil. He distinguishes two kinds of this spreading, one a gradual creeping of the electricity over the surface of the glass, Art. 300, and the other instantaneous, Art. 307.

He attempted to check the first kind by varnishing the glass plates and by enclosing their edges in a thick frame of cement, but he found very little advantage in this method, and finally adopted the plan of performing all the operations of the experiment as quickly as possible, so as to allow very little time for the gradual spreading of the electricity.

He next investigated the instantaneous spreading of electricity on the glass near the edge of the coating. He noticed that at the instant of charging the plate in the dark, a faint light could be seen all round the edges. He also observed that after charging

SPREADING OF ELECTRICITY.

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and discharging a coated plate of glass many times without cleaning it, a narrow fringed ring of dirt could be traced all round the coating, the space between this ring and the coating being clean, and in general about inch broad.

He also observed that the flash of light was stronger the first or second times of charging a plate than afterwards.

To determine how much the capacity of a coated plate was increased by this spreading of the electricity, he compared the capacity of a plate with a circular coating with that of the same plate with a new coating of nearly the same area, but cut into strips, so that its perimeter was very much greater than that of the circular coating.

In this way he found that if we suppose a strip of uniform breadth added to the coating all round its boundary, the capacity of this coating, supposing the electricity not to spread, will be equal to that of the actual coating as increased by the spreading of the electricity. The most probable breadth of this strip he found to be 0.07 inch for thick glass and 0·09 for thin.

When this correction was applied to the areas of the coatings of the different coated plates, the computed charges of plates made of the same kind of glass were found to be very nearly in the same ratio as their observed charges.

But the observed charges of coated plates were found to be always several times greater than the charges computed from their thickness and the area of their coatings, the ratio of the observed charge to the computed charge being for plate glass about 8.2, for crown glass about 8.5, for shellac about 447, and for bees' wax about 3.5. Thus Cavendish not only anticipated Faraday's discovery of the Specific Inductive Capacity of different substances, but measured its numerical value in several substances.

The values of the specific inductive capacity of various substances as determined by different modern observers are compared with those found by Cavendish in the table in Note 27.

To make it certain, however, that the difference between the observed and calculated capacities of coated plates really arose from the nature of the plate and not from some error in the theory,