36 
PROFESSOR TYNDALL ON THE DIAMAGNETIC FORCE, ETC. 
driven from the cores, whereas when the arrangement was that shown in fig. 41, it 
was drawn towards them. 
But it does not sufficiently express the facts to say that the bar is capable of two 
different states of excitement ; it must be added, that both states exist simultaneously 
in the excited bar. We have already proved that the state necessary for the action 
of one pole is not that which enables an opposite pole to produce the same action ; 
hence, when the two ends of the bar are attracted or repelled, at the same time, by 
two opposite poles, it is a proof that these two ends are in different states. But if 
this be correct, we can test our conclusion by reversing one of the poles; the direc- 
tion of its force being thereby changed, it ought to hold the other pole in check and 
prevent all motion in the bar. This is the case : if, in any one of the instances cited, 
the polarity of either of the cores be altered ; if the south be converted into a north, 
or the north into a south pole, thus making both poles of the same quality, the repul- 
sion of the one is so nearly balanced by the attraction of the other, that the bar re- 
mains without motion towards either of them. 
To carry the argument a step further, let us fix our attention for an instant upon 
fig. 40. The end of the bar nearest to the reader is repelled by a south pole ; the 
same end ought to be attracted by a north pole. In like manner, the end of the bar 
most distant from the reader is repelled by a north pole, and hence the state of that 
end ought to fit it for attraction by a south pole. If, therefore, our reasoning be cor- 
rect, when we place a north pole opposite to the lower end of the bar, and on the same 
side of it as the upper north pole, and a south pole opposite the upper end of the bar 
and on the same side of it as the lower south pole, the simultaneous action of these 
four poles ought to be more prompt and energetic than when only two poles are used. 
This arrangement is shown in Plate III.: the two poles to the right of the bismuth 
bar must be of the same name, and the two to the left of the bar of the opposite qua- 
lity. If those to the right be both north, those to the left must be both south, and 
vice versd. The current reverser for the magnets appears in front, that for the helix 
is hidden by the figure. The above conclusion is perfectly verified by experiments 
with this apparatus, and the twofold deflection of the bismuth bar is exhibited with 
remarkable energy*. 
The bar used in these cases is far heavier than those commonly made use of in 
experiments on diamagnetism ; but the dimensions stated do not mark the prac- 
tical limit of the size of the bar. A solid bistriuth cylinder, 14 inches long and 1 inch 
in diameter, was suspended in a helix 6*7 inches long, I S inch internal diameter, 
4 inches external diameter, and composed of copper wire 0*1 of an inch in thickness : 
when a current of twenty cells was sent through the helix, and the magnets (only 
* These experiments, and almost all the others mentioned in this memoir, may be exhibited in the lecture 
room. By attaching indexes of wood to the bars of bismuth, and protecting the indexes from air currents by 
glass shades, the motions may he made visible to several hundreds at once. See a description of a Polymagnet, 
Phil. Mag. June 1855. — J. T. 
