PROFESSOR TYNDALL ON THE DIAMAGNETIC FORCE, ETC. 
29 
Preserving' the helix current as in the last experiment ; when the polarity of the 
magnet was reversed, the deflection was that shown in fig-. 15. 
Interrupting both, and reversing the current in the helix ; when the magnet was 
excited as in the last experiment, the deflection was that shown in fig. 16. 
In a paper on the Polarity of Bismuth* published in the Philosophical Magazine, 
Ser. 4. vol. ii., and in Poggendorff’s Annalen, vol. Ixxxvii., an experiment is recorded 
in which the deportment exhibited by fig. 1 1 of the present series was obtained. In a 
recent memoir on the same subject, M. v. Feilitsch'|~ 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 currents ; for 
immediately afterwards a deflection in the opposite direction was observed, which 
deflection proved to be the permanent one. 
I have repeated the experiment here referred to with all possible 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 suspended within the helix. The effect, as will 
be seen, is not one regarding which a mistake could be made on account of its minute- 
ness : operating delicately, and choosing a suitable relation between the strength of 
the magnet and that of the spiral ;}:, on sending a current through the latter as in 
fig. 11, the bar was deflected so forcibly that the limit of its first impulsion reached 120° 
on the graduated circle underneath. The permanent deflection of the bar amounted to 
60° in the same direction, and hence the deportment could in no wise 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. 
* From the notices of this paper which have appeared in the continental journals, I am obliged to infer that 
it is in some respects obscurely written. The conclusion I intended to express is that bismuth possesses a 
polarity opposed to that of iron. — J. T. 
t Poggendoeff’s Annalen, vol. xcii. p. 395. 
I In most of these experiments the spiral was excited by ten cells, the magnet by two. 
