L00 ON FARADAY'S LINES OF FORCE. 



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 Faradayt 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 resist- 

 ance. 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 direct appli- 

 cation. 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, XLvn. 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, 



* Exp. Re$. (2429), (3320). See Weber, Poggendorff, LXXXVII. p. 145. Pro TyndaU, Phil. 

 Trans. 1856, p. 237. 



t Exp. Ret. (2836), <kc. 



