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The induction theory of magnetism, introduced by Faraday, is now looked upon by all physicists as correct. The older theory which assumes the existence of magnetic fluids covering the ends of the magnet is in some cases mathematically simpler, and is for this reason often made use of. This, however, is apt to breed confusion as to the true nature of the induction or polarization in any given case. The difficulty Tyndall experienced in accepting Faraday's views as to diamagnetism, is accounted for by the fact that he was thinking in terms of the fluid theory, while Faraday was considering the magnetic polarization in the diamagnetic substance.
The object of this paper is to insist again upon the distinction between these two theories, and at the same time to consider some points in the induction theory itself. A number of diagrams will be described which illustrate the different aspects of the induction theory. They show how the molecules of the magnet are supposed to be polarized, and how this polarization is continued in the surrounding ether. The tubes of force, or "polarization tubes," and the equipotential surfaces are drawn in each case according to Maxwell's method. The figures must be revolved about the horizontal axis, so that the lines drawn will trace out, some the bounding surfaces between the tubes, and others the equipotential surfaces.