658 



CONSTANTS OF MAGNETISATION. 



were sufficiently accounted for on the assumption that magnetic 

 bodies move towards points where the force is maximum, and dia- 

 magnetic towards the points of minimum force. The opinion of 

 physicists was at first divided between these two views of Faraday, 

 but this divergence ceased from the time when Sir W. Thomson 

 showed that these two explanations merge into each other (395). 



We shall describe two experiments by Tyndall* which show 

 clearly the inverse polarity of bismuth. 



A bismuth bar is suspended horizontally in the axis of a mag- 

 netising coil A, so that its ends are opposite poles of the contrary 

 kind of two electromagnets. The deflection of the bar is the inverse 

 of that of an iron bar; the current of the coil A develops then in 



J 



A 



13 



n i s 







i' n' 



13 



II 



13 



Fig. 251. 



the bismuth a pole N at its right, and a pole S at its left. The 

 deflection changes its direction with the direction of the current 

 in the coil A, which proves that the magnetisation of bismuth is 

 due mainly to the field of this coil, and that the influence of the 

 electromagnets is only secondary. 



The principle of the second experiment is due to Weber, f 

 The apparatus consists of two long vertical coils AB, A'B' (Fig. 251), 

 placed side by side. By an endless wire passing over two pulleys, 

 two bismuth bars ab t a'b' can be displaced in opposite directions 

 along the axis of each coil. Two magnetised needles ns, and V, in 



* TYNDALL. Phil. Trans, p. 24. 1855. 



t WEBER. Electrodyn. Maasbcstim., Abh. dcr Konigl. Sac A. Gesellschaft, 

 Vol. i., p. 483. 1852. 



