270 Prof. Tyndall on the Nature of ike Force by which 



upon the bismuth bar observed. M. v. Feilitseh has attempted 

 a similar experiment to that here described^ but without success : 

 when, however, sufficient power is combined with sufficient deli- 

 cacy, the success is complete, and the most perfect mastery is 

 obtained over the motions of the bar. 



The helix above described is the one which I have found most 

 convenient for the experiments ; various other helices, however, 

 were ti-ied with a result equally certain, if less energetic. The 

 one first made use of was 4 inches long, 3 inches exterior dia- 

 meter, and three-quarters of an inch interior diameter, with wire 

 one-fifteenth of an inch in thickness, the bar being suspended 

 by a fibre which passed through a slit in the helix : sending 

 through this helix a cui'rent from a battery of 10 cells, and ex- 

 citing the cores by a current from ] cell, the phjenomena of re- 

 pulsion and attraction were exhibited with all desirable precision. 



I shall now proceed to describe the results obtained by opera- 

 ting in the manner described. The bismuth bar being suitably 

 suspended, a current was sent through the helix, so that the 

 direction of the current in the upper hulfvi&s that indicated by 

 the arrow in fig. 40, PI. IV. On exciting the magnet, so that the 

 pole N was a north pole and the pole S a south pole, the ends of 

 the bar of bismuth were repelled. The final position of the bar 

 was against the side of the helix most remote from the magnets : 

 it is shown by dots in the figure. 



By means of the reverser R the current was now sent through 

 the helix in the direction shown in fig. 41 : the bar promptly 

 left its position, crossed the space in which it could freely move, 

 and came to rest as near the magnets as the side of the helix 

 would permit it. // ivas manifesthj attracted by the magnets. 



Permitting the current in the helix to flow in the last direc- 

 tion, the polarity of the cores of sott iron was reversed : we had 

 then the state of things sketched in fig. 42 ; the bismuth bar 

 instantly loosed from the position it formerly occupied, receded 

 from the magnet, and took up finally the position marked by 

 the dots. 



After this new position had been attained, the current through 

 the helix m as reversed : the bar promptly sailed across the field 

 towards the magnets, and finally came to rest in the dotted 

 position, fig. 43. In all these cases, when the bar was freely 

 moving in any direction, under the operation of the forces acting 

 upon it, the reversion either of the current in the helix or of the 

 polarity of the cores arrested the motion ; approach was converted 

 into recession, and recession into approach. 



The ends of the helix in these experiments were not far from 

 the ends of the soft iron cores ; and it might therefore be sup- 

 l)osed that the action was due to some moditicalion of the cores 



