302 



Mr. G. 0. Simpson on the Electrical 



the bismuth, S, 



containing 



A Wheatstone-bridge (fig. 1) 

 was balanced for steady currents, and then an alternating 

 current supplied instead. The "bismuth E.M.F." which 

 this alternating current set up caused an alternating difference 



J D/W»£"LL' 

 Cell 



of potential across the galvanometer-arm AB in phase with, 

 and of the same wave-form as, itself. An adjustable contact- 

 maker, M, on the dynamo-shaft closed the galvanometer-arm 

 for an instant at any required phase of an alternation ; the 

 result being a deflexion of the galvanometer which varied in 

 magnitude and sign with the phase of the "make." The 

 " make " having been set at a known phase, a potentiometer, 

 connected to the ends of the galvanometer-arm, AB, was then 

 adjusted until there was no deflexion of the galvanometer, 

 in which case the potentiometer-reading gave the value of 

 the "bismuth E.M.F." at the instant of the make. 



Curve I. (fig. 2) shows the form of the " bismuth E.M.F." 

 obtained in this way. 



In order to compare this "bismuth E.M.F." with the 

 curve of the current producing it, the bismuth spiral was 



