PROFESSOR TYNDALL ON THE DIAMAGNETIC FORCE, ETC. 
23 
of magnetization induced in them by the influencing magnet. This conclusion is in 
no way invalidated by the recent researches of M. Plucker, on the law of induction 
in paramagnetic and diamagnetic bodies, but on the contrary derives support from 
his experiments. With current strengths which stand in the ratio of 1 : 2, M. Plucker 
finds the repulsion of bismuth to be as 1 : 3-62, which, though it falls short of the ratio 
of 1 : 4, as the law of increase according to the square of the current would have it, 
is sufficient to show that the bismuth was not passive, but acted the part of an in- 
duced diamagnet in the experiments. In the case of the soft iron itself M. Plucker 
finds a far greater divergence ; for here currents which stand in the ratio of 1 : 2 pro- 
duce attractions only in the ratio of 1 : 2’76. 
2. Duality of Diamagnetic Excitement. 
Having thus safely established the fact, that diamagnetic bodies are repelled, in 
virtue of a certain state into which they are cast by the influencing magnet, the 
next step of our inquiry is ; — Will the state evoked by one magnetic pole facilitate, 
or prevent, the repulsion of the diamagnetic body by a second pole of an opposite 
quality? If the force of repulsion were an action on the mass, considered as ordinary 
matter, this mass, being repelled by both the north and the south pole of a magnet, 
when they operate upon it separately, ought to be repelled by the sum of the forces 
of the two poles where they act upon it together. But if the excitation of diamag- 
netic bodies be of a dual nature, as is the case with magnetic bodies, then it may be 
expected, that the state excited by one pole will not facilitate, but on the contrary 
prevent, the repulsion of the mass by a second opposite pole. 
To solve this question the apparatus sketched in fig. 5a. Plate II. was made use of. 
AB and CD are two helices of copper wire 12 inches long, of 2 inches internal, and 
of 5^ inches external diameter. Into them fit soft iron cores 2 inches thick ; the 
cores are bent as in the figure, and reduced to flat surfaces 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 gh 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 experimenter’s 
power to exeite 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 presented 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 ends of the semicylinders were turned so as to present the blunted apex of a cone to the mass of 
bismuth. 
