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Viscount Mahon, afterwards Lord Stanhope, a man of great ingenuity and fertility in invention, a pupil of Le Sage of Geneva, and the inventor of the printing press which bears his name, published in 1779 his Principles of Electricity. The theory developed in this book is that
“A positively electrified body surrounded by air will deposit “upon all the particles of that Air which shall come successively into “contact with it, a proportional part of its superabundant Electricity, “ By which means, the Air surrounding that body will also become “positively electrified : that is to say, it will form round that positive "body, an electrical atmosphere, which will likewise be positive.” (p. 7.)
“That the electrical Density of all such Atmospheres decreases, “ when the distance from the charged Body is increased.” (p. 14.)
He then proceeds to determine the law of the density of the electrical atmosphere, as it depends on the distance from the charged body. He assumes that if a cylinder with hemispherical ends is placed in the electrical atmosphere of a charged body, the density of the electricity at any part of the cylinder will depend on the density of the electrical atmosphere in contact with it.
He also shows by experiment that if the cylinder is insulated, and originally without charge, it does not become charged as a whole by being immersed in the electrical atmosphere of a charged body. Hence, when the electricity of the cylinder is disturbed, the whole positive charge on one portion of the surface is numerically equal to the whole negative charge on the other portion.
Now if the density (on the cylinder) were inversely as the distance from the charged body, a transverse section of the cylinder whose distance from the charged body is the geometric mean of the distances of the ends, would divide the charge into two equal parts (both of course of the same kind of electricity), but if the density were inversely as the square of the distance, the distance of the section which would bisect the charge would be the harmonic mean of the distance of the ends. In all this he tacitly confounds the point of bisection of the charge with the neutral point.
He then shows by experiment that the actual position of the neutral point agrees sufficiently well with the harmonic mean, but not with the geometric mean, and from this he concludes (p. 65),
"Consequently, it evidently appears, from what was said above, “that the Density of the Electricity, of the electrical Atmosphere (in “which the said Body A, B was immersed) was in the inverse Ratio of the
square of the Distance."
It is evident from this that Lord Mahon was entirely ignorant of everything which Cavendish had done.
About the close of the century Dr Thomas Young, whose acquaintance with all branches of seience was as remarkable for its extent as for its profundity, says of this neutral point:
“ It was from the situation of this point that Lord Stanhope first “ inferred the true law of the electric attractions and repulsions, although “ Mr Cavendish had before suggested the same law as the most probable "supposition." (Lecture LIII.)
The same writer, in his “Life of Cavendish,” in the Supplement to the Encyclopædia Britannica, gives the following account of the first paper on electricity.
“3. An Attempt to explain some of the principal Phenomena of Electricity by means of an Elastic Fluid. (Phil. Trans. 1771, p. 584.) “Our author's theory of electricity agrees with that which had been "published a few years before by Æpinus, but he has entered more
minutely into the details of calculation, showing the manner in wbich “the supposed fluid must be distributed in a variety of cases, and
explaining the phenomena of electrified and charged substances as they are actually observed. There is some degree of unnecessary complication from the great generality of the determinations : the “ law of electric attraction and repulsion not having been at that time “fully ascertained, although Mr Cavendish inclines to the true sup
position, of forces varying inversely as the square of the distance : “this deficiency he proposes to supply by future experiments, and leaves “it to more skilful mathematicians to render some other parts of the “theory still more complete. He probably found that the necessity “ of the experiments, which he intended to pursue, was afterwards “superseded by those of Lord Stanhope and M. Coulomb; but he “had carried the mathematical investigation somewhat further at a “later period of his life, though he did not publish his papers; an “ omission, however, which is the less to be regretted, as M. Poisson, " assisted by all the improvements of modern analysis, has lately treated “the same subject in a very masterly manner. The acknowledged im"perfections, in some parts of Mr Cavendish's demonstrative reasoning,
“have served to display the strength of a judgment and sagacity still more admirable than the plodding labours of an automatical calcu
One of the corollaries * seems at first sight to lead to a mode “of distinguishing positive from negative electricity, which is not justi"fied by experiment; but the fallacy appears to be referable to the very comprehensive character of the author's hypothesis, which requires some little modification to accommodate it to the actual circumstances of the electric fluid, as it must be supposed to exist in “nature."
No man was better able than Dr Young to appreciate the scientific merits of Cavendish, and it is evident that he spared no pains in obtaining the data from which he wrote this sketch of his life, yet this account of his electrical researches shows a complete ignorance of Cavendish's unpublished work, and this ignorance must have been shared by the whole scientific world.
Dr Young, as it appears from the above extract, was aware of the existence of unpublished papers by Cavendish relating to electricity, but he supposed that these papers were entirely mathematical, and that “ he probably found that the necessity of the experiments which he intended to pursue was afterwards superseded by those of Lord Stanhope and M. Coulomb."
We now know that the unpublished mathematical papers were entirely subsidiary to the experimental ones, and it is plain from Art. 95 that Cavendish had actually made some of his experiments before the paper of 1771, and that all those on electrostatics were completed before the end of 1773.
The favourable reception which Lord Stanhope's very interesting and popular experiments met with may have influenced Cavendish not to publish his own, but his estimate of their value as a foundation for a theory of electricity may be gathered from the fact, that in his “Thoughts concerning Electricity," which appears to be his earliest writing on the subject, he devotes two pages (Arts. 195—198) to the refutation of the very theory of electric atmospheres which is the basis of Lord Stanhope's reasoning; whereas in the paper of 1771, which contains his more matured views, he does not even allude to that theory.
Art. 49 and Note 1.
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 19.4 inches in diameter with his globe of 12.1 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.