Resistance of Bismuth to Alternating Currents, 307 



the cell was replaced by a dynamo ; a commutator, on the 

 dynamo shaft, was introduced into the galvanometer-arm, 

 and a shunt was connected to the ratio-arm. When this had 

 been done it was quite easy to keep the bridge balanced for 

 steady currents, the necessary change in the resistance of 

 the ratio-arm being made by means of the shunt. 



Bis. 6. 



Two series of experiments were made : the first dealing 

 with frequencies between 10 and 70 per sec, in which the 

 alternating current was obtained from the dynamo mentioned 

 above ; and the other dealing with frequencies between 3 and 

 10 per sec, the alternating current in this case being obtained 

 from a liquid commutator which gave a current having an 

 accurate sine wave form. 



In performing an experiment it was found best to set the 

 brushes of the commutator in the galvanometer circuit to 



r 



commutate at a known angle </>, and then to find the ratio ^ 



for different frequencies. 



The results from such a series of experiments are shown in 

 fig. 6; from these curves another series was drawn, figs. 7 



r 



& 8, in which the values of tt for a given frequency are 



