1022 
PROFESSOR J. A. EWING AND MISS H. G. KLAASSEN 
the two layers had the same number of turns, and the number was the same on both 
rings. The two layers, both of primary and of secondary, could be connected either 
inductively or non-inductively. They were each wound uniformly round the whole of 
the ring. On one ring they were connected inductively, and were respectively in 
series with the non-inductive primary and secondary of the other ring. Hence all 
direct thermal action of primary and secondary currents was the same for both rings. 
The magnetic reversals were caused by passing through the primary circuit the current 
from an alternating dynamo, which could be run at various speeds, and the strength 
Fig. 24. 
of this current was adjusted, by means of a liquid rheostat, E.^^, in its circuit, to secure 
that the magnetization should have the same limiting value when the secondary circuit 
was closed (through the resistance Kg) as when it was open. To test the constancy in 
the limit of magnetization, a voltmeter, V^, of special construction was used, which was 
connected to a special induction coil of a few turns wound on each ring. The voltmeter 
was of the hot wire type ; it consisted of a piece of very fine platinum wire, stretched 
between supports, but deflected sideways, after the manner of the wire in Professors 
Perry and Ayrton’s voltmeter, by a light spring pulling transversely in the middle of 
the wire’s length. The middle portion of the wire stood in the field of a microscope, 
and its transverse movements were read on a micrometer scale in the eye-piece. This 
was found to form a very convenient means of reading alternating electromotive forces 
of one or two volts and under. Owingf to the small section of the wire and the 
freedom from friction which was secured by having no mechanical indicating gear, the 
instrument responded quickly and exactly to change of electromotive force, and allowed 
the necessary adjustment of the primary alternating current to be made with great 
nicety. Tn most of the experiments a Cardew voltmeter, Vg, was also connected across 
the terminals of the primary circuit. Its response to small changes was much less rapid 
