HISTORY.] ELECTRICITY 15 not yet rightly comprehend. Among the older labours in this field we may mention those of Pliicker and Hittorf, De la Rive, Riess, Gassiot, and Varley. But even as we write cur knowledge of the subject is extending, and we refrain from referring to more modern results; for historical sketching a difficult task in any case is unsafe in an open field like this, where some apparently insignificant fact may contain the germ of a great discovery. We may here mention the experiments of Wheatstone on the velocity of electricity, valuable less for the results he obtained than for the ingenious application of the rotating mirror, then used for the first time, which has since been applied with much success in the study of the electric discharge. One of the greatest names in electrical science is that of Riess. In his classical research on the heating of wires by the discharge from a battery of Leyden jars, he did for elec tricity of high potential what Joule did for the voltaic current. The electro-thermometer which he used in these researches was an improvement on the older instruments of Kinnersley and Harris. Riess repeated and extended the experiments of Coulomb, and effected many improvements in the apparatus for electrostatical experiments. His Reibungs- dedricitdt is a work of great value, and was for long the best book of reference open to the experimental student. Happily we have now another in the recently published work of M. Mascart. Sir William Thomson revolutionized experimental elec tricity by introducing instruments of precision. Chief among these are his quadrant and absolute electrometers. His portable electrometer and water-dropping apparatus are instruments of great value to the meteorologist in the study of atmospheric electricity, a science which he has done much in other ways to forward. Besides this, we owe to him many valuable suggestions for electrical apparatus and ex perimental methods, some of which have been carried out by his pupils. sctro- The theory of statical electricity has made great progress tical since Poisson s time. Among its successful cultivators we
- r *- may mention Murphy (Electricity, 1833), and Plana (1845).
The latter went over much the same ground as Poisson, extending his results. It was, however, by Gresn (Essay on The Application of Mathematical Analysis to the Theories of Electricity and Magnetism, 1828 ; or Mathematical Papers, edited by N. M. Ferrers), a self-taught mathemati cian, that the greatest advances were made in the mathema tical theory of electricity. " His researches," as Sir William Thomson has observed, " have led to the elementary pro position which must constitute the legitimate foundation of every perfect mathematical structure that is to be made from the materials furnished in the experimental laws of Coulomb. Not only do they afford a natural and complete explanation of the beautiful quantitative experiments which have been so interesting at all times to practical electri cians, but they suggest to the mathematician the simplest and most powerful methods of dealing with problems which, if attacked by the mere force of the old analysis, must have remained for ever unsolved." One of the simplest appli cations of these theorems was to perfect the theory of tho Leyden phial, a result which (if we except the peculiar action of the insulating solid medium, since discovered by Faraday) we owe to his genius. He has also shown how an infinite number of forms of conductors may be invented, so that the distribution of electricity in equilibrium on each may be expressible in finite algebraical terms, an immense stride in the science, when we consider that the distribu tion of electricity on a single spherical conductor, an unin fluenced ellipsoidal conductor, and two spheres mutually influencing one another, were the only cases solved by Poisson, and indeed the only cases conceived to be solvable by mathematical writers. The work of Green, which con tained these fine researches, though published in 1828, had escaped the notice not only of foreign, but even of British mathematicians ; and it is a singular fact in the history of science that all his general theorems were re discovered by Sir William Thomson, Chasles and Sturm, and Gauss (see Reprint of Thomson s papers). Sir Wil liam Thomson, however, pushed his researches much further than his fellow-labourers. He showed that the experimental results of Sir William Snow Harris, which their author had supposed to be adverse to the theory of Coulomb, were really in strict accordance with that theory in all cases where they were sufficiently simple to be sub mitted to calculation. He was guided in his earlier in vestigations by an analogy between the problems involved in steady flux, of heat and the equilibrium of electri city on conductors. He showed in 1845 how the pecu liar electric polarization discovered by Faraday in di electrics, or solid insulators subjected to electric force, is to be taken into account in the theory of the Leyden iar, so as to supply the deficiency in Green s investigations. We also owe to Sir William Thomson new synthetical methods of great elegance and power. The theory of electric images, and the method of electric inversion founded thereon, constitute the greatest advance in the mathema tical theory of electrostatics since the famous memoir of Green. These he has applied in the happiest manner to the demonstration of propositions which had hitherto re quired the resources of the higher analysis, and he has also found by means of them the distribution oil a spherical bowl, a case of great interest in the theory of partially closed conductors, which had never been attacked or even dreamt of as solvable before. The work of Professor Clerk Maxwell on Electricity and Magnetism, which appeared in 1873, has already exerted great in fluence on the study of electricity both in England and on the Continent. In it are fully given his valuable theory of the action of the dielectric medium. He regards the electrical forces as the result of stress in the medium, and calculates the stress components which will give the observed forces, and at the same time account for the equilibrium of the medium. The striking discovery re cently made by Mr Kerr of Glasgow, of the effect on polarized light exerted by a piece of glass under the action of strong electric force, is of great import ance in connection with Maxwell s theory, and realizes a cherished expectation of Faraday, of whom Maxwell is the professed exponent. We must allude here once more to Maxwell s electromagnetic theory of light, the touchstone of which is the proposition that in transparent media, whose magnetic inductive capacity is very nearly equal to that of air, the dielectric capacity is equal to the square of the index of refraction for light of infinite wave length. Although, as perhaps was to be expected, owing to disturbing influences such as heterogeneity, this proposition has not been found in good agreement with experiment in the case of solids, yet for liquids (Silow, Pogg. Ann,, civ. clviii.) and gases (Boltzmann, Hid. civ.) the agreement is so good as to lead us to think that the theory contains a great part of the whole truth. In the earlier stages of the science several units were in- Absolut* troduced for the measurement of quantities dealt with in units, electricity. As examples of these we may mention the wire of Jacobi, and the mercury column of Siemens, a metre long, with a section of a square millimetre, which at given temperatures furnished units of resistance ; the DanielPs cell, which furnished the unit of electromotive force, the chemical unit of current intensity, <tc. All these units were perfectly arbitrary, and there was no con nection of any kind between them. The introduction of
a rational system of unitation, based on the fundamental