34 
PROFESSOR TYNDALL ON THE DIAMAGNETIC FORCE, ETC. 
connected tog-ether that the same current excited both, thus developing the same 
magnetic strength in the poles P P'. From the ends of the spirals proceeded wires to 
the current reverser R', and thence to a second battery of considerably less power 
than the former. By means of the reverser R' the polarity of the cores could be 
changed ; P' could be converted from a south pole to a north pole, at the same time 
that P was converted from a north pole to a south pole. Lastly, by a change of the 
connexions between the two spirals, the cores could be so excited as to make the poles 
of the same quality, both north or both south. 
The diameter of the cylindrical space, within which the bismuth bar was suspended, 
was such as to permit of a free play of the ends of the bar through the space of an 
inch and a half. Having seen that the bar swung without impediment, and that its 
axis coincided as nearly as possible with the axis of the helix, a current from the 
battery was sent through the latter. The magnetism of the cores P and P' was then 
excited, and the action upon the bismuth bar observed, M. v. Feilitsch has attempted 
a similar experiment to that here described, but without success: when, however, 
sufficient power is combined with sufficient delicacy, the success is- complete, and the 
most perfect mastery is obtained over the motions of the bar. 
The helix above described is the one which I have found most convenient for the 
experiments ; various other helices, however, were tried with a result equally certain, 
if less energetic. The one first made use of was 4 inches long, 3 inches exterior dia- 
meter and three-quarters of an inch interior diameter, with wire one-fifteenth of an 
inch in thickness, the bar being suspended by a fibre which passed through a slit in 
the helix: sending through this helix a current from a battery of 10 cells, and ex- 
citing the cores by a current from 1 cell, the phenomena of repulsion and attraction 
were exhibited with all desirable precision. 
I shall now proceed to describe the results obtained by operating in the manner 
described. The bismuth bar being suitably suspended, a current was sent through 
the helix, so that the direction of the current in the upper Aa^was that indicated by 
the arrow in fig. 40. On exciting the magnet, so that the pole N was a north pole 
and the pole S a south pole, the ends of the bar of bismuth were repelled. The final 
position of the bar was against the side of the helix most remote from the magnets : 
it is shown by dots in the figure. 
By means of the reverser R the current was now sent through the helix in the 
direction shown in fig. 41 : the bar promptly left its position, crossed the space in 
which it could freely move, and came to rest as near the magnets as the side of the 
helix would permit it. It was manifestly attracted by the magnets. 
Permitting the current in the helix to flow in the last direction, the polarity of the 
cores of soft iron was reversed : we had then the state of things sketched in fig. 42 ; 
the bismuth bar instantly loosed from the position it formerly occupied, receded from 
the magnet and took up finally the position marked by the dots. 
After this new position had been attained, the current through the helix was re- 
