PROFESSOR R. THRELFALL AND MR. .J. H. D. BREARLFT 
$8 
determines a large part of the mechanical design. The coils must be carried sepa¬ 
rately, insulated on brass holders, for the nicety required is not to be obtained by the 
use of any softer or less rigid material. The framework of the instrument which holds 
the coils is made in two parts, which are scraped to fit ; no screws or clamps are 
necessary to hold the two halves of the frame together : one half, containing half the 
coils, can be lifted up and carried away from the other half, the plane of separation 
being the plane containing the magnetic system. In order that the adjustment of the 
magnet system to the coils may be perfectly made, it is necessary to be able both to 
look through the axial holes of the coils and also into the narrow space (1‘5 millim.) 
separating the two systems of coils. The method of mounting the coils will be clear 
from the drawings and description (Plate 1), but it is necessary here to indicate the 
general procedure. The coils, when wound, were embedded concentrically in rings of 
ebonite, a little paraffin being poured in and fastened both to the coil and ebonite 
by means of a hot wire. Four bars of fused quartz had been previously fastened as 
spokes into the ebonite—regarded as a hub—and by means of a centering apparatus 
devised by Mr. Cook, each coil with its ebonite and quartz was laid in a proper 
position in its coil carriage. This was a matter requiring much ingenuity, and was 
accomplished by means of the adjustable catches into which the ends of the quartz 
rods drop, and which are screwed to the carriage. The coil being in the centering 
apparatus, the catches of the carriage are adjusted to the quartz spokes; the catches 
are then made fast to the carriage, and then the quartz is made permanently fast to 
the catches by means of a little plaster of Paris. The quartz spokes are like nails, 
i.e., they have heads, and are fastened to the ebonite rings by being pushed through 
them from inside before the coil is centred and cemented by paraffin, which also, of 
course, cements the head of the quartz spoke. Each coil carriage is screwed to the 
frame by separate screws, which allow a little latitude for final adjustment to the 
magnetic system when this is suspended. The detail of the whole of the devices for 
adjusting coils insulated by quartz rods, we owe to Mr. Cook, and we are not sure 
that the system would succeed with less perfect workmanship than he put into the 
construction. The arrangement proved in practice to be everything that could be 
desired. 
(2.) The apparatus carrying the coils and magnet system must be absolutely separate 
and independent of the parts carrying the controlling magnets. This is a condition 
of the first importance —however good all the other arrangements may be, they will fail 
in practice unless this condition is fulfilled. We have tried over and over again with the 
controlling system more or less connected to the suspended system, but have never 
had any success with such an arrangement, however massive the brass work. This 
condition may be regarded as the result of all our experience. At one time we tried 
supporting the coil system on rubber bungs mounted with more or less elaboration, 
and on discs of rubber mounted in different ways ; but observations with a mercury 
trough convinced us that with our concrete and ashphalt block floors, the rubber always 
