16 
PROFESSOR TYNDALL ON THE DIAMAGNETIC FORCE, ETC. 
Let the strength of the influencing south pole increase from M to wM ; then, 
assuming the magnetism of the body under influence to increase in the same ratio,' 
the strength of the above-mentioned north pole will become wM', and the attraction, 
expressed by the product of both, will be 
w^MM' ; 
that is to say, the attraction of a body magnetized by influence, and whose mag- 
netism varies as the strength of the influencing magnet, is proportional to the square 
of the strength of the latter. 
Here then is a mark of distinction between those bodies which have their power 
of exhibiting magnetic phenomena conferred upon them by the magnet, and those 
whose actions are dep'endent upon some constant property of the mass : in the latter 
case the resultant action will be simply proportional to the strength of the magnet, 
while in the former case a different law of action will be observed*. 
The examination of this point lies at the very foundation of our inquiries into the 
nature of the diamagnetic force. Is the repulsion of diamagnetic bodies dependent 
merely on the mass considered as ordinary matter, or is it due to some condition 
impressed upon the mass by the influencing magnet ? This question admits of the 
most complete answer either by comparing the increase of repulsion with the increase 
of power in the magnet which produces the repulsion, or by comparing the attrac- 
tion of a paramagnetic body, which we know to be thrown into an unusual condi- 
tion, with the repulsion of a diamagnetic body, whose condition we would ascertain. 
Bars of iron and bismuth, of the same dimensions, were submitted to the action of 
an electromagnet, which was caused gradually to increase in power ; commencing 
with an excitation by one cell, and proceeding up to an excitation by ten or fifteen. 
The strength of the current was in each case accurately measured by a tangent galva- 
nometer. The bismuth bar was suspended between the two flat poles, and, when the 
magnet was excited, took up the equatorial position. The iron bar, if placed directly 
between the poles, would, on the excitation of the magnetism, infallibly spring to 
one of them ; hence it was removed to a distance of 2 feet 7 inches from the centre 
of the space between the poles, and in a direction at right angles to the line which 
united them. The magnet being excited, the bar was drawn a little aside from its 
position of equilibrium and then liberated, a series of oscillations of very small am- 
plitude followed, and the number of oscillations accomplished in a minute was care- 
fully ascertained. Tables III. and IV. contain the results of experiments made in 
the manner described with bars of iron and bismuth of the same dimensions. 
* This test was first pointed out in a paper on the Polarity of Bismuth, Phil. Mag. Nov. 1851, p. 333. I have 
reason, however, to know that the same thought occurred to M. PojGGExdorff previous to the publication of 
my paper. — J. T. 
