ELECTRO-MAGNETISM. 



27 



itself be influenced by the polarity of 

 those portions of the magnet which 

 lie near the axis, and the rotatory ten- 

 dency impressed upon it will produce 

 the rotation of the magnet, which will, 

 of course, be carried along with it. On 

 the latter supposition, it will correspond, 

 in all its circumstances, with the experi- 

 ment 73, in which the conducting 

 body is urged to rotate by the influence 

 of a magnetic pole situated within it : 

 excepting only, that in the former case 

 the magnet and the conducting body 

 were one and the same, while in the 

 latter they were different ana separate. 

 Mr. Faraday has shown, however, that 

 the circumstance of the magnet and 

 conductor being immoveably joined to- 

 gether makes no difference in the results. 

 Thus let the magnet M, represented in 

 section, Jig. 47, be loaded at its lower 

 end with a platina weight, and fixed at 

 its upper end on a piece of card or wood, 

 having two branches of a strong wire, 

 W W, descending from its upper edge 



Fig. 47. 



munications with a voltaic battery by 

 means of the two cups containing mer- 

 cury. This experiment is important, 

 inasmuch as it appears to show that the 

 action is the same, whether the magnet 

 from which it proceeds be in motion or 

 at rest. We shall have occasion, how- 

 ever, in a future part of this treatise, to 

 point out another mode of explanation 

 arising out of a different view of the 

 subject. 



(77.) On the other hand, when a hollow 

 cylinder of metal, balanced on a point 

 on the upper end of a vertical axis of 

 wood, and its lower edge dipping into a 

 trough of mercury, is acted upon by one 

 of the poles of a magnet placed on the 

 outside, and brought near it, as shown 

 in the section /#. 48, where M is the 

 magnet applied to the cylinder C, ba- 

 lanced in the wooden stand S, the rota- 

 tory force is very feeble, compared with 

 that which takes place when the mag- 



Kg. 48. 



along its two vertical edges, and termi- 

 nating below in points: so that the 

 whole may float, in a vertical position, 

 in a vessel full of mercury, from the bot- 

 tom of which a wire proceeds, support- 

 ing the cup N ; another cup, P, being 

 placed upon the upper edge of the wires 

 W W. The whole moveable part of 

 this apparatus will rotate by the trans- 

 mission of an electric current through 

 the wires, on making the proper corn- 



net acts from within the cylinder, j The 

 reason is, that the tendencies to motion 

 of those portions of the moveable con- 

 ductor which are most remote from the 

 magnet, and of those which are nearest 

 to it, are in opposite directions with re- 

 spect to the centre of motion ; and, if 

 the conductor be cylindrical, and the 

 current equally distributed on every 

 side of it, must always exactly coun- 

 terbalance one another. This will be 

 evident when it is considered that, 

 although these latter portions are, in 

 consequence of their greater proximity to 

 the magnet, acted upon more strongly, 

 this advantage is compensated by the 

 greater extent of the portion on the re- 

 mote side, which is acted upon more 

 feebly. But ^his equilibrium will not 

 obtain if, as generally happens, the elec- 

 tric current be unequally distributed. 

 If, for instance, it pass along one side 

 only, the cylinder will revolve when the 

 magnet is brought opposite to it on that 

 side. 



