178 Prof. Tyndall on the Diamagnetic Force, ^c 



along the line ef, so that when the two semicylindrical ends are 

 placed together, they constitute a cylinder of the same diameter 

 as the cores within the helices*. In front of these poles a bar of 

 pure bismuth yh was suspended by cocoon silk ; by imparting a 

 little torsion to the fibre, the end of the bar was caused to press 

 gently against a plate of glass ik, which stood between it and the 

 magnets. By means of a current reverser the polarity of one of 

 the cores could be changed at pleasure ; thus it was in the experi- 

 menter's power to excite the cores, so that the poles PP' should 

 be of the same quality, or of opposite qualities. 



The bar, being held in contact with the glass by a very feeble 

 torsion, a current was sent round the cores, so that they pre- 

 sented two poles of the same name to the suspended bismuth ; 

 the latter was promptly repelled, and receded to the position 

 dotted in the figure. On interrupting the current it returned 

 to the glass as before. The cores were next excited, so that two 

 poles of opposite qualities acted upon the bismuth ; the latter 

 remained perfectly unmovedt- 



This experiment shows that the state, whatever it may be, into 

 which bismuth is cast by one pole, so far from being favourable 

 to the action of the opposite pole, completely neutralizes the 

 effect of the latter. A perfect analogy is thus established between 

 the deportment of the bismuth and that of soft iron under the 

 same circumstances ; for it is well known that a similar neutral- 

 ization occurs in the latter case. If the repulsion depended upon 

 the abstract strength of the poles, without reference to their 

 quality, the repulsion, when the poles are of opposite names, 

 ought to be greater than when they are alike ; for in the former 

 case the poles are greatly strengthened by their mutual inductive 

 action, while, in the latter case, they are enfeebled by the same 

 cause. But the fact of the repulsion being dependent on the 

 quality of the pole, demonstrates that the substance is capable 

 of assuming a condition peculiar to each pole, or in other words, 

 is capable of a dual excitation J. The experiments from which 



* The ends of the semicj'linders were turned so as to present the blunted 

 apex of a cone to the mass of bismuth. 



t A shorter bar of bismuth than that here sketched, with a light index 

 attached to it, makes the repulsion more evident. It may be thus rendered 

 visible throughout a large lecture-room. 



X Since the above was written, the opinion has been expressed to me, 

 that the action of the unlike poles, in the experiment before us, is " diverted " 

 from the bismuth upon each other, the absence of repulsion being due to 

 this diversion, and not to the neutralization of inductions in the mass of 

 the bismuth itself. Many, however, will be influenced by the argument 

 as stated in the text, who would not accept the interpretation referred to in 

 this note ; I therefore let the argument stand, and hope at no distant day 

 to return to the subject. — J. T., May 5, 1855. 



