50 Mr. S. C. Laws on the Magnetic Susceptibility 
for determining the susceptibility of bismuth, but regards 
this as the best of the four. 
For my purpose this method has also the advantage that 
the specimens are required in the form of cylindrical rods, so 
that precisely the same specimens are used in these experi- 
ments as had been employed in the investigation referred to 
above ; also the amount of apparatus required is comparatively 
small and the experiments could be quickly carried out. 
In the experiments the coil was 40 cms. in length and 
consisted of 4158 turns of No. 16 B.W.G. copper wire 
arranged in 21 layers upon an ebonite cylinder 2°70 cms. in 
external diameter, and having stout flanges of vulcanised 
fibre screwed on to its ends. 
In order to measure the force acting on a rod this was 
suspended from one arm of a sensitive balance, the coil being 
arranged so that the rod hung freely along the axis and in 
such a position that, when the current was sent through the 
coil, the rod assumed a position with half its length pro- 
jecting beyond the coil. In order to secure this adjustment 
the coil was mounted on a tripod provided with levelling 
screws, Above the coil, and moving with this, was fixed a 
scale by means of which it could be ascertained when the 
rod occupied the required position (fig. 1). 
Care was also taken to arrange the coil vertical, with the 
rod situated along its axis. 
When a current was sent through the coil it was found 
that each of the specimens examined showed a decrease in 
weight, so that they were all diamagnetic, 
As this decrease of weight was never more than 3 milli- 
grammes, methods had to be adopted to render visible a much 
smaller deflexion of the beam of the balance than could be 
indicated with the ordinary pointer. 
Accordingly, the double suspension method due to Lord 
Kelvin, and used by Poynting in his experiments on the 
gravitation constant *, was employed. For this purpose the 
pointer belonging to the balance was removed and replaced 
by one made of brass and bent forwards at a short distance 
from its free end at right angles to the beam of the balance. 
One end of a single short fibre of raw silk was fixed to this 
horizontal free end, the other end of the fibre being attached 
to the end of a fixed hcrizontal brass rod situated a few 
millimetres in front of the end of the pointer. © 
From the silk fibre was suspended, by means of two light 
copper hooks, a light plane mirror, 1 cm. in diameter. 
* Poynting, Phil. Trans. 182 a, p. 572 (1891). 
