Page:On Faraday's Lines of Force.pdf/26

180 in the reverse direction in diamagnetic bodies. In fact we have not as yet come to any facts which would lead us to choose any one out of these three theories, that of lines of force, that of imaginary magnetic matter, and that of induced polarity. As the theory of lines of force admits of the most precise, and at the same time least theoretic statement, we shall allow it to stand for the present.

Theory of Magnecrystallic Induction.

The theory of Faraday with respect to the behaviour of crystals in the magnetic ﬁeld may be thus stated. In certain crystals and other substances the lines of magnetic force are conducted with different facility in different directions. The body when suspended in a uniform magnetic ﬁeld will turn or tend to turn into such a position that the lines of force shall pass through it with least resistance. It is not difﬁcult by means of the principles in (28) to express the laws of this kind of action, and even to reduce them in certain cases to numerical formulae. The principles of induced polarity and of imaginary magnetic matter are here of little use; but the theory of lines of force is capable of the most perfect adaptation to this class of phenomena.

Theory of the Conduction of Current Electricity.

It is in the calculation of the laws of constant electric currents that the theory of ﬂuid motion which we have laid down admits of the most dire ct application. In addition to the researches of Ohm on this subject, we have those of M. Kirchhoff, ''Ann. de Chim''. XLI. 496, and of M. Quincke, XLVII. 203, on the Conduction of Electric Currents in Plates. According to the received opinions we have here a current of ﬂuid moving uniformly in conducting circuits, which oppose a resistance to the current which has to be overcome by the application of an electro-motive force at some part of the circuit. On account of this resistance to the motion of the ﬂuid the pressure must be different at different points in the circuit. This pressure. which is commonly called electrical tension,