428 Prof. Tyudall on the Polymagnet. 



against it, the free ends of the four cores being thus in contact. 

 The same current being sent through both magnets, \vc have 

 the mutual attraction of two electro-magnets instead of the 

 attraction of an electro-magnet for a mass of soft iron, as 

 supposed in the last experiment. The arrangement just de- 

 scribed is indeed precisely that devised by M. Pouillet in the 

 construction of a powerful electro-magnet for the faculty of 

 sciences at Paris. To the cross-piece of the second magnet a 

 ring is also attached, from which weights can be suspended. 



4. The cross-pieces may be removed by withdrawing the 

 screws g and h, and the spirals may be made use of singly with 

 their corresponding bar-magnets. As two wires surround each 

 coil, one of them may be used to exhibit the induced currents 

 developed by the other. The phenomena of the extra-current 

 may also be studied, and the remarkable effect produced by con- 

 necting the two ends of one of the wires, or the spark of the 

 extra-current in the other, may be exhibited. 



The milled head a, fig. 7, can be screwed off, and the rod 

 ac pushed downwards into the case ; the helix in the centre can 

 also be readily lifted out of its position and removed from the 

 glass case — we will suppose this done. The two electro-magnets 

 alone are now within the case, and the view is uninterrupted, 

 which would not be the case if the helix had been permitted to 

 remain. 



The fibre hanging from the groove d, fig. 7, can be so arranged 

 that any substance attached to it shall hang between the move- 

 able masses of soft iron which surmount the electro-magnets, 

 and the same arrangement can be made for the fibre suspended 

 from the groove d'. 



But a body suspended between the moveable masses of soft iron 

 would be hidden by these masses from the audience, and hence to 

 render the motions of the body visible the following expedient was 

 adopted : — fig. 8a represents a thin index of ivory about 4 inches 

 long, and shaped as in the figure ; from the centre depends the 

 stem dc, which is terminated by a tongs-shaped arrangement 

 which can clasp the body to be submitted to experiment ; to the 

 right of the index a section of the little ivory pliers, by a plane 

 passing through the stem at right angles to i n, is given ; the 

 stem is slit up to a, so as to allow of the pliers being opened to 

 receive the body to be examined, which they then clasp in virtue 

 of the elasticity of the ivory. The stem de is of such a length, 

 that when the body is in the centre of the space between the 

 poles, the index i n is seen above them ; and, as the index fol- 

 lows all the motions of the body underneath, these motions are 

 recognized by all who see the index. 



5. If an ordinary magnetic bar, sufficiently feeble, be suspended 



