PROFESSOR TYNDALL ON THE DIAMAGNETIC FORCE, ETC. 
35 
versed : the bar promptly sailed across the field towards the magnets, and finally 
came to rest in the dotted position, fig. 43. In all these cases, when the bar was 
freely moving in any direction, under the operation of the forces acting upon it, the 
reversion either of the current in the helix or of the polarity of the cores arrested the 
motion ; approach was converted into recession and recession into approach. 
The ends of the helix in these experiments were not far from the ends of the soft 
iron cores ; and it might therefore be supposed that the action was due to some modi- 
fication of the cores by the helix, or of the helix by the cores. It is manifest that the 
magnets can have no 'permanent effect upon the helix ; the current through the latter, 
measured by a tangent galvanometer, is just as strong when the cores are excited as 
when they are unexcited. The helix may certainly have an effect upon the cores, and 
this effect is either to enfeeble the magnetism of the cores or to strengthen it ; but if 
the former, and the bar were the simple bismuth which it is when no current operates 
on it, the action, though weakened, v)ould still be repulsive, and if the latter, the increase 
would simply augment the repulsion. The fact, however, of the ends of the bar being 
attracted, proves that the bar has been thrown into a peculiar condition by the current 
circulating in the surrounding coil. Changing the direction of the current in the coil, 
we find that the self-same magnetic forces which were formerly attractive are now 
repulsive ; to produce this effect the condition of the bar must have changed with 
the change of the current ; or, in other words, the bar is capable of accepting two 
different states of excitement, which depend upon the direction of the current. 
In order, however, to reduce as far as possible the action of the helix upon the 
cores, I repeated the experiments with the small helix referred to in fig. 6, page 25. It 
will be remembered that this helix is but an inch in length, and that the bismuth bar 
is 6^ inches long. I removed the magnets further apart, so that the centres of the cores 
were half an inch beyond the ends of the bismuth bar, while the helix encircled only 
an inch of its central portion: in this position, when the helix was excited, there was 
no appreciable magnetism excited by it in the dormant cores ; at least, if such were 
excited, it was unable to attract the smallest soft iron nail. Here then we had cores 
and helix sensibly independent of each other, but the phenomena appeared as before. 
The bar could be held by the cores against the side of the helix, with its ends only a 
quarter of an inch distant from the ends of the cores ; on reversing either current the 
ends instantly receded, but the recession could be stopped by again changing the 
direction of the current. With a tranquil atmosphere, and an arrangement for re- 
versing the current without shock or motion, the bar obeyed in an admirable manner 
the will of the experimenter, and, under the operation of the same forces, exhibited 
all the deflections sketched in figs. 40, 41, 42 and 43. 
The motion of the bar cannot be referred to the action of induced currents. The 
bar was brought into the centre of the hollow cylinder in which it swung and held 
there ; the forces were all in action, and therefore all phenomena of induction passed ; 
the arrangement of the forces being that shown in fig. 40, on releasing the bar it was 
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