264 Prof. Tyndall on the Nature of the Force by which 



deportment exhibited by fig. 1 1 of the present series was ob- 

 tained. In a recent memoir on the same subject, M. v. FeiUtsch* 

 states that he has sought this result in vain. Sometimes he 

 observed the deflection at the moment of closing the circuit, but 

 conceived that it must be ascribed to the action of induced cur- 

 rents ; for immediately afterwards a deflection in the opposite 

 direction was observed, which deflection proved to be the perma- 

 nent one. 



I have repeated the experiment here referred to with all pos- 

 sible care ; and the result is that described in the remarks which 

 refer to fig. 11. This result agrees in all respects with that 

 described in my former paper. To enable myself, however, to 

 appeal to quantitative measurement, a small graduated circle 

 was constructed and placed underneath the bar of bismuth sus- 

 pended within the helix. The efiiect, as will be seen, is not one 

 regarding which a mistake could be made on account of its 

 minuteness : operating delicately, and choosing a suitable rela- 

 tion between the strength of the magnet and that of the spiral t, 

 on sending a current through the latter as in fig. 11, the bar 

 was deflected so forcibly that the limit of its first impulsion 

 i*eached 120° on the graduated circle underneath. The perma- 

 nent deflection of the bar amounted to 60° in the same direc- 

 tion, and hence the deportment could in no mse be ascribed to 

 the action of induced currents, which vanish immediately. Before 

 sending the current through the helix, the bar was acted on by 

 the magnet alone, and pointed to zero. 



Though it was not likely that the shape of the poles could 

 have any influence here, I repeated the experiment, using the 

 hemispherical ends of two soft iron cores as poles : the result 

 was the same. 



A pair of poles with the right- and left-hand edges rounded 

 off", showed the same deportment. 



A pair of poles presenting chisel edges to the helix showed 

 the same deportment. 



Various other poles were made use of, some of which appeared 

 to correspond exactly with those figured by M. v. Feilitsch ; but 

 no deviation from the described deportment was observed. To 

 test the polarity of the magnet, a magnetic needle was always at 

 hand : once or twice the polarity of the needle became reversed, 

 which, had it not been noticed in time, would have introduced 

 confusion into the experiments. Here is a source of error against 

 which, however, M. v. Feilitsch has probably guarded himself. 

 Some irregularity of crystalline structure may, perhaps, have 



* PoggendorfFs Annalen, vol. xcii. p. 395. 



t In most of these experiments the spiral was excited by ten cells, the 

 magnet by two. 



