24 
PROFESSOR TYNDALL ON THE DIAMAGNETIC FORCE, ETC. 
The cores were next excited, so that two poles of opposite qualities acted upon the 
bismuth ; the latter remained perfectly unmoved*. 
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 circum- 
stances ; for it is well known that a similar neutralization 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 condi- 
tion peculiar to each pole, or in other words, is capable of a dual excitation-f”. The 
experiments from which these conclusions are drawn are a manifest corroboration of 
those made by M. Reich with steel magnets. 
If we suppose the flat surfaces of the two semicylinders which constitute the ends 
of the cores to be in contact, and the cores so excited that the poles P and P' are of 
different qualities, the arrangement, it is evident, forms a true electro-magnet of the 
horseshoe form ; and here the pertinency of a remark made by M. Poggendorff, with 
his usual clearness of perception, becomes manifest ; namely, that if the repulsion of 
diamagnetic bodies be an indifferent one of the mass merely, there is no reason why 
they should not be repelled by the centre of a magnet, as well as by its ends. 
3. Separate and joint action of a Magnet and a Voltaic Current on Paramagnetic and 
Diamagnetic Bodies. 
In operating upon bars of bismuth with the magnet, or the current, or both com- 
bined, it was soon found that the gravest mistakes might be committed, if the question 
of structure was not attended to ; that it is not more indefinite to speak of the volume 
of a gas without giving its temperature, than to speak of the deportment of bismuth 
without stating the relation of the form of the mass to the planes of crystallization. 
Cut in one direction, a bar of bismuth will set its length parallel to an electric 
current passing near it; cut in another direction, it will set its length perpendicular 
* 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. 
t 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, how- 
ever, 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., 
.5th May, 1855. 
