through the window, the two pieces of foil could be made to appear of the same intensity. The sum of the direct and reflected light is then equal for both pieces of foil. The uncoated piece must reflect just as much more light than the coated as it is deficient in the amount of direct light it can transmit. This is in accordance with Kirchhoff's law-that is, that in a heated space a bundle of rays made up of direct and reflected rays from a surface show the same peculiarities that a bundle of rays from a dark hot body would show. Mr. St. John utilized this idea by bringing a cold porcelain cylinder into the neighbourhood of the pieces of foil; the bare platinum could then be quickly distinguished from the surrounding hot walls, and appeared darker than the coated platinum. As soon as the rod took the temperature of the oven the field of view appeared uniformly bright. Such are some of the considerations to which I am led by a study of the electric oven; they must be regarded not as final considerations, but merely as attempts to penetrate into the mysteries of the sun. CHAPTER XXII. WHAT IS ELECTRICITY? THE old fluid theories of electricity can be said to have been buried in a common grave with the theories of caloric and phlogiston, and Faraday's researches give the deathblow to the old theory of action at a distance (J. J. Thomson). I have endeavoured to show in the preceding chapters that in the phenomena of the transformations of energy there is a great field of investigation which will repay the student to study and to work in. If we can not discover what electricity is, we can ascertain its relations to light and heat. The human mind, however, is far-reaching, and loves to frame hypotheses and theories, and perhaps no subject is fuller of theories than that of electricity. If we abandon Maxwell's electro-magnetic theory of light, we find that we must choose between a great number of rival theories-the theories of Ampère, Grassman, Stefan, Korteweg, Neumann, Gauss, Keber, Riemann, and Clausius-in which the actions in a medium between magnets and electrical circuits are not considered. Moreover, the various rotational phenomena of magnetism and electricity are not fully embraced in these theories. These theories, moreover, do not consistently connect the manifestations of light, heat, and electricity, and most of them are not founded on the doctrine of the conservation of energy. Prof. J. J. Thomson, in a report on electrical theories,* divides the theories into the following classes: "1. Theories in which the action between elements of currents is deduced by geometrical considerations, combined with assumptions which are not explicitly, at any rate, founded on the principle of the conservation of energy. This class includes the theories of Ampère, Grassman, Stefan, and Korteweg. "2. Theories which explain the action of currents by assuming that the forces between electrified bodies depend upon the velocities and accelerations of the bodies. This class includes the theories of Gauss, Weber, Riemann, and Clausius. "3. Theories which are based upon dynamical considerations, but which neglect the action of the dielectric. This class contains L. E. Neumann's potential theory and Helmholtz's extension of it. "4. C. Neumann's theory. "5. Theories which are based upon dynamical considerations, and which take into account the action of the dielectric. This class includes the theories of Maxwell and Helmholtz." Prof. Thomson criticises the various theories, and shows "that they can be divided into two great classes, according as they do or do not take into account the action of the dielectric surrounding the various conductors in the field." According to the potential theories of L. E. Neumann and Helmholtz, in an unclosed circuit there are forces which arise from the discontinuity at the ends of the circuit. Shiller's + experiments * Report of the British Association, Aberdeen, 1885. show, however, that this potential theory is wrong "if we neglect the action of the dielectric and assume that the current stops at the end of the circuit." Shiller also showed that Ampère's theory fails for open circuits, and that Grassman's and Clausius's theories must be wrong as well as Ampère's and Korteweg's, for Shiller's experiments proved that the dielectric must be taken into account. Maxwell's great theory is full of the intimations which we all have in regard to the probable relation between the varied transformations of energy. It rests, however, upon the hypothesis of the existence of displacement currents in a non-conductor, and the existence of these currents has never been satisfactorily shown by experiment. When a Leyden jar, for instance, is discharged by connecting the outer coating to the inner by a wire, it is supposed by Maxwell that displacement currents of electricity occur in the glass of the jar, which is the dielectric which separates the outer tin-foil coating of the jar from the inner coating. These currents are called displacement currents, to distinguish them from the apparent current in the wire which discharges the jar. The electrical state on the coatings of the jar is supposed to be rapidly displaced to and fro by the oscillations of the jar. The conduction currents in the wire are transformed into heat. While the displacement currents do not manifest this transformation, they are supposed, however, to exert magnetic influences like ordinary conduction currents. They not only are called into existence in the glass or dielectric of the jar, but they also appear in the air surrounding the jar. They * Poggendorf's Annalen, vol. clix, p. 456. are instantaneous currents, and depend on the rate of change of the electro-motive force, or difference of potential between the coatings of the Leyden jar, and also upon the substance of the dielectric, whether it be glass, or rubber, or air. I have said that the existence of these displacement currents has never been proved by experiments which are free from criticism. The most satisfactory investigation is that of Hertz, who apparently showed that displacement currents in a large pile of books, and in large masses of other dielectrics, exerted a magnetic effect upon the electric waves emanating from an oscillator. The position of the little circle of wire (N, Fig. 47) constituting his exploring resonator was influenced by the proximity of a large solid dielectric, such as a block of paraffin, and he attributed the disturbance to Maxwell's displacement currents in this dielectric. Prof. J. J. Thomson remarks: * "The most pressing need in the theory of electro-dynamics seems to be an experimental investigation of the question of the continuity of these dielectric currents. We have experimental proof that they exist (?), but we do not know whether Maxwell's assumption that they always form closed circuits with the other currents is true or not. If Maxwell's assumption should turn out to be true, we should have a complete theory of electrical action." What shall we therefore answer to the question, What is electricity? Must we reply, Ignoramus ignorabimus (We are ignorant, and we shall remain ignorant)? We have already strong grounds for believing that we live in a medium which conveys to-andfro or periodic movements to us from the sun, and * Report on Electrical Theories. |