66 
LORD RAYLEIGH OH THE VALUE OF THE BRITISH 
Ill the original apparatus the ring is very light, in fact scarcely strong enough to 
stand the forces to which it is subjected in winding on the wire. In order to avoid 
this defect, and also on account of its larger size, the new ring was made very massive. 
Cast solid, with lugs at the ends of what was to be in use the horizontal diameter, 
it was cut into two equal parts along a horizontal plane. The two parts were then 
insulated from one another by a layer of ebonite, and firmly joined together again at 
the lugs by bolts and nuts, after which the grooves, &c., were carefully turned. As it 
was intended to use two coils of wire in perpendicular planes, two rings were prepared. 
The smaller ring fitted into the larger, the end pieces passing through holes along the 
vertical diameters of both. But for a reason that will presently be given, only the 
larger ring was used in the present experiments. 
In the spring of 1881 the larger ring was wound in Messrs. Elliotts' shop under 
the superintendence of Dr. Schuster and myself, and the necessary measurements 
were taken. On mounting the apparatus a few days later in the magnetical room of 
the Cavendish Laboratory, and making preliminary trials, we were annoyed by finding 
a very perceptible effect upon the suspended magnet even when the wire circuit was 
open. The currents thus indicated might have been due to a short circuit in the wire, 
or more probably (considering that the wire was triple covered, and that the winding 
had been carefully done) had their seat in the ring itself. Experiment showed that 
the insulation between the two parts of the ring, as well as between the wire and each 
part, was very good, so that no currents could travel round the entire circumference; 
but on consideration it appeared not unlikely that currents of sufficient intensity 
might be generated in those parts of the ring which lie nearest to the ebonite layer. 
The width of the ring (in the direction of its axis) was 4 inches, and the least 
thickness—that at the bottom of the grooves—about f- inch, so that the operative 
parts may be compared to four vertical plates § inch thick, 4 inches broad, and (say) 
6 inches high. In these plates currents will be developed during the rotation, whose 
plane is perpendicular to that of the currents in the wire. 
The unwished for currents could doubtless have been much diminished by saw cuts 
in a vertical plane extending a few inches upwards and downwards from the insulating 
layer, but it appeared scarcely safe to assume that the ring would retain its shape 
under such treatment. It would have been wiser to have tried the effect of spinning 
the ring alone before winding on the wire, but we were off our guard from the fact 
that the old ring gave no perceptible disturbance. 
Theory having shown that these currents, if really formed in the manner supposed, 
could be satisfactorily allowed for, we decided to proceed with the experiment. At the 
worst, the differential effect between wire circuit closed and wire circuit open could 
only be in error by a quantity depending upon the square of the speed, and therefore 
capable of elimination upon the evidence of the spinnings themselves; while if the view 
were correct that the disturbing currents were principally in a plane perpendicular to 
that of the wire, even the correction for induction would not be much affected. A 
