Current and Potential Difference Anal 



ics. 



213 



dhg equal to one another, we can, no doubt, make the differ- 

 ence of phase in the two currents passing through the dyna- 

 mometer exactly the same as the difference in phase between 

 the current through ah and the P.D. at the terminals of 

 ab; but we cannot make the currents through the dyna- 

 mometer coils independent of the rate of alternation. Hence, 

 if this instrument be employed for measuring the power 

 given to ab in the way shown in figure 10, it must be 

 calibrated for each rate of alternation of the current. 



But although this defect exists in the employment of 

 Mr. Bimington's high-resistance split dynamometer for the 

 measurement of power, it can be used without error for 

 measuring the phase-angle between two P.D.s in series by 

 a method analogous to that employed by Mr. Blakesley for 

 measuring the phase-difference between two currents in parallel. 



Fig. 11 shows Mr. Blakesley's method : the dynamo- 



Fk. 11. 



nrrrvs. 



meters 1 and 2 give respectively the mean squares of the 

 currents in the two circuits, and the dynamometer 3 the mean 

 product of the two currents ; and from the three readings we 

 have, as is now well known, 



cos 6 



V 



{Square of reading of 3 

 Reading of 1 x Beading of 2' 



where 6 is the angle of lag. 



Fiff. 12. 



Now let fg and hj be two circuits in series (fig. 12), 

 and let it be required to find the angle of phase- difference 



