104 



Mr. G. C. Simpson on the Electrical 



curves, it will be seen that within the limits of experimental 

 error they are of the same wave-form, but that the E.M.F. 

 curve lags about 120° behind the current curve. 



These experiments having shown that an alternating cur- 

 rent with a frequency of 20 passing through bismuth in a 

 strong magnetic field produces an effect which may be repre- 

 sented by an alternating E.M.F. in the bismuth 120° behind 

 the current, it remained to examine how the magnitude and 

 phase of this " bismuth E.M.F/'' depend on the frequency of 

 alternation. 



For this purpose a different method of experimenting was 

 adopted, the theory of which is as follows : — 



Let an alternating E.M.F. be applied to a Wheatstone- 

 bridge ADBC (fig. 3) containing the bismuth spiral in the 



Fig. 3. 



arm BD. and previously balanced for steady currents, so that 

 the E.M.F. across BD is of the form E sin 6. Then the 

 "bismuth E.M.F/' produced in the spiral, lagging a behind 

 the current, will be of the form e sin (0—a) — the largest 

 value of e obtained in the experiments being only 3 per cent, 

 of E, the current through the bismuth is practically in phase 

 with the E.M.F. applied to B and D. There will now be a 

 current through the galvanometer-arm AB, which being pro- 



portional to e may be written K ^ 

 on the resistances in the bridge. 



sin (0 — a) . 



K depending 



