Nov. 28, 1878] 



NATURE 



83: 



spot marks the place of the pole, we should perceive that 

 the systems of circles round the current and of rays 

 round the pole mutually disturbed each other, and that 

 the figure was consequently unsymmetrical. Round one 

 half of the figure the lines coalesce ; round the other they 

 repel each other, and stream away. Applying the notions 

 we have already obtained, we see that the result will be a 





Fig. g. 



force tending to make the pole and the current rotate 

 round each other. This, we know, was shown by Fara- 

 day himself to be the case, for when one was fixed and the 

 other free to move, the one rotated round the other. Carry 

 on the idea one stage further, and make the current run 

 up through the plate at the precise point where the pole 



they would certainly rotate the central region round or> 

 itself. The corresponding fact exists in another of 

 Farada/s discoveries : that a magnet can be made to. 

 rotate round its own axis, under the influence of a current 

 running up it through one of its poles. 



One experiment more will close for the present out 



Fig. II. 



Study in magnetism. We know that a rod of iron be- 

 comes a magnet when we wind a spiral of m ire round it 

 and send a current through the wire. There must be 

 some relation between the iron bar and the coils of wire : 

 what is it? Let us investigate this also by locking at 

 the distribution of the lines of force within the coiL 



Fig. 10. 



^^^^A J?^^^* ^s. Let it run up through the magnet. The 

 held we obtam (Fig. 11) shows us neither the rays of 

 the magnet nor the circles of the current, but a set of 

 beautiful spirals unwinding from a common centre. \Vhat 

 Tf"t ^ K °^'°" can we deduce from this remarkable figure ? 

 " the branches of the spiral could shorten themselves 



Fig. 12. 



Take a plate of glass or a piece ot card and bore eight 

 holes through it, as in Fig. 12, and wind a corkscrew of 

 wire in and out ; then lay a little bit of thin, soft iron 

 down the middle. We see by the lines of force, w'.ien 

 the current is passed, that the iron becomes a strong 

 magnet, but that the wires at the same time are mag- 



