ON THE ELECTRICAL PROPERTIES OF PURE SULPHUR. 
89 
did more harm than good. At present the instrument is mounted on a hard 
smooth slab of concrete, which reposes on a sandstone pillar resting on the concrete 
on which the floors are laid. The coil system is mounted on a sheet of ebonite 
supported by insulators on the concrete slab, and the controlling magnet system is 
supported by leaden pillars passing through the ebonite without touching it, and also 
resting on the concrete slab. The natural stiffness of the slab, or rather its inertia, 
is much increased by the fact that it carries the magnetic shield of cast iron weighing 
about three hundred pounds. This materially lessens any tendency there might be 
to produce disturbance in the suspended system by adjusting the controlling magnets. 
The Magnetic Control. —The whole secret of success lies in this. Instead of 
regarding the control as a subsidiary part of the apparatus, it must be regarded 
as the most important part, and must be capable of the finest adjustment. We have 
found it advisable to use the controlling magnets at such a distance that the residual 
field is due to them, and not to the earth. In this case, small changes in the 
direction of the earth’s field produce less effect than when the residual field is that 
due to the magnetic action of the earth. We have a simple but important improve¬ 
ment to report in the matter of control. It turns out that when the sensitiveness is 
very high, the uniformity of the magnetic field becomes very important. When the 
field is not sufficiently uniform, the following fatal effect is observed. We will 
suppose that the earth’s field is in excess, and that the magnets are in the magnetic 
meridian. The control magnet, we wall suppose, is lowered gradually ; finally, a point 
is reached at which the period is fairly long, and the suspended part, if nearly at 
rest, behaves apparently well ; now let it be caused to vibrate (in practice, the 
control can never be lowered without causing the suspended magnets to vibrate 
more or less). It is always noticed that if the excursion rises beyond a certain very 
small value, then the suspended magnet swings round perhaps 70° or 80°, and 
takes up a new position of greater stability than the old one. In order to bring 
it back the control magnets have to be rotated, with the invariable effect of 
making the suspended magnet swing past the north and south position, and take 
up a more or less symmetrical position on the other side. Attempts to bring the 
needles back by moving external magnets, or working damping arrangements, 
seldom succeed, and in the end the controlling magnet has usually to be raised, and 
the tiresome business begun afresh. We have wasted many days over this untoward 
phenomenon. Suppose, however, that the difficulty is apparently surmounted and 
the image got on the scale and fairly steady, and observations begun. It is always 
found that after a time the needle begins to drift, and finally goes over to one of the 
side positions. This was first noted by Mr. Pollock with the old galvanometer in 
1889. The cause of these effects is not far to seek. The field produced by a simple 
control magnet is never uniform. In such a long system as ours, and with the 
controlling magnet at the top, the lower suspended magnet is directed by the earth’s 
field, and the upper one by the control, the centre ones being probably in a nearly 
MDCCCXCVI.—A. N 
