Page:On Faraday's Lines of Force.pdf/24

178 If the conduction of the dielectric is perfect or nearly so for the small quantities of electricity with which we have to do, then we have the case of (24). The dielectric is then considered as a conductor, its surface is a surface of equal potential, and the resultant attraction near the surface itself is perpendicular to it.

Theory of Permanent Magnets.

A magnet is conceived to be made up of elementary magnetized particles, each of which has its own north and south poles, the action of which upon other north and south poles is governed by laws mathematically identical with those of electricity. Hence the same application of the idea of lines of force can be made to this subject, and the same analogy of ﬂuid motion can be employed to illustrate it.

But it may be useful to examine the way in which the polarity of the elements of a magnet may be represented by the unit cells in ﬂuid motion. In each unit cell unity of ﬂuid enters by one face and ﬂows out by the opposite face, so that the ﬁrst face becomes a unit sink and the second a. unit source with respect to the rest of the ﬂuid. It may therefore be compared to an elementary magnet, having an equal quantity of north and south magnetic matter distributed over two of its faces. If we now consider the cell as forming part of a system, the ﬂuid ﬂowing out of one cell will ﬂow into the next, and so on, so that the source will be transferred from the end of the cell to the end of the unit tube. If all the unit tubes begin and end on the bounding surface, the sources and sinks will be distributed entirely on that surface, and in the case of a magnet which has what has been called a solenoidal or tubular distribution of magnetism, all the imaginary magnetic matter will be on the surface.

Theory of Paramagnetic and Diamagnetic Induction.

Faraday has shown that the effects of paramagnetic and diamagnetic bodies in the magnetic ﬁeld may be explained by supposing paramagnetic bodies to