22G 
MESSRS. R, T. GLAZEBROOK AND J. M. DODDS ON THE 
To place the second coil with its plane parallel to the first we used three cylinders 
of brass, the ends of which had been carefully turned so as to be at right angles to 
their axes, while the lengths of the cylinders were as nearly as possible the same. 
These three cylinders were placed vertically on the upper side of the coil A, one 
above each of the arms of the tripod, and the coil B rested on them ; its plane thus 
was as nearly as may be parallel to that of A. 
To bring its centre into the axis a second tripod exactly similar to the first was 
placed in an inverted position over it, and the coil was moved about until the flange 
pieces fitted inside the inner surface of the annulus as before. In this manner the 
coils were adjusted to the required position. 
The following account of the precautions used in the winding, and the methods 
employed to measure the constants of the coils, is quoted from a letter of Professor 
Chrystal’s, addressed to one of the authors of this paper (R. T. G.) :— 
“ The coils, stand, &c., were constructed after working drawings made by myself 
under the supervision of Professor Maxwell. The immediate end in view in con¬ 
structing the coils was the determination of the ohm, and this of course influenced 
the design of the stand. It was proposed ultimately 3 to use the coils as a standard 
instrument for producing a uniform magnetic field in which to determine galvanometer 
constants and the like. 
“ The coils were wound by myself and the then mechanic at the laboratory, 
Mr. Fulcher. The coils were mounted for this purpose by placing between the three 
armed supports ”—the tripods mentioned above—“ which were then braced together 
and mounted on an axle and stand. During the winding constant tests were taken for 
the insulation between the wire and the metal channel. This was the main difficulty, 
and wherever the slightest defect was discovered the wire was unwound for a little 
way and paraffin paper and paraffin used. It was found absolutely necessary, in order 
to secure good insulation, to cover the bottom of the channel with a ribbon of silk 
drawn through melted paraffin. The number of turns in each layer was separately 
