Faraday. 219 



to ordinary induced magnetism in all respects, except that the 

 direction of the induced polarity is reversed. It was accepted 

 by other investigators, notably by W. Weber, Pliicker, Eeich, 

 and Tyndall ; but was afterwards displaced from the favour of 

 its inventor by another conception, more agreeable to his peculiar 

 views on the nature of the magnetic field. In this second 

 hypothesis, Faraday supposed an ordinary magnetic or para- 

 magnetic* body to be one which offers a specially easy passage 

 to lines of magnetic force, so that they tend to crowd into 

 it in preference to other bodies ; while he supposed a dia- 

 magnetic body to have a low degree of conducting power for 

 the lines of force, so that they tend to avoid it. " If, then," he 

 reasoned,f "a medium having a certain conducting power occupy 

 the magnetic field, and then a portion of another medium or 

 substance be placed in the field having a greater conducting 

 power, the latter will tend to draw up towards the place of 

 greatest force, displacing the former." There is an electrostatic 

 effect to which this is quite analogous ; a charged body attracts 

 a body whose specific inductive capacity is greater than that of 

 the surrounding medium, and repels a body whose specific 

 inductive capacity is less; in either case the tendency is to 

 afford the path of best conductance to the lines of force .J 



For some time the advocates of the "polarity" and 

 " conduction " theories of diarnagnetism carried on a contro- 

 versy which, indeed, like the controversy between the adherents 

 of the one-fluid and two-fluid theories of electricity, persisted 

 after it had been shown that the rival hypotheses were mathe- 

 matically equivalent, and that no experiment could be suggested 

 which would distinguish between them. 



Meanwhile new properties of magnetizable bodies were being 

 discovered. In 1847 Julius Pliicker (b. 1801, d. 1868), Professor 

 of Natural Philosophy in the University of Bonn, while 

 repeating and extending Faraday's magnetic experiments, 



* This term was introduced by Faraday, Exp. Res., 2790. 

 t Exp. es., 2798. 



J The mathematical theory of the motion of a magnetizable body in a non- 

 uniform field of force was discussed by "W. Thomson (Kelvin) in 1847. 



