Page:Elementary Text-book of Physics (Anthony, 1897).djvu/325

§ 269] electrical forces are due to stresses in the medium between the electrified bodies serves very well in expressing the results of experiment, but it gives no information about the origin or cause of the stresses in the medium. Faraday, who originated the theory, apparently thought that they arise from the electrification by induction of the separate particles or molecules of the medium in such a way that they resemble, so far as their external action is concerned, the magnetic molecules in Weber's theory of magnetism. This view was not held consistently even by its author, and cannot be accepted if we remember that electrical actions take place through vacuum. Maxwell conceived of electricity as distributed everywhere in space, and considered the charging of a body as a displacement of the electricity in the region around it in one sense if the body is charged positively, in the opposite sense if charged negatively. Conductors offer no resistance to such a displacement, but in dielectrics the displacement is resisted by an action which Maxwell called electrical elasticity. This mode of describing the electrical field is satisfactory so long as the field is considered in equilibrium, but becomes difiicult of application when movements of charges occur in the field. J. J. Thomson has shown that all the phenomena of the electrical field may be described in terms of the motions or interactions of tubes of force, one of which is supposed to be connected with each atom of matter in the field. Thomson gives no mechanical explanation of the properties which these tubes must be assumed to have, only saying that "the analogies between their properties and those of the tubes of vortex motion irresistibly suggest that we should look to the rotary motion in the ether for their explanation."