ELECTRICAL MEASUREMENT BY ALTERNATING CURRENTS 307 



include this resistance. However, the value of R" will not affect the 

 first adjustment much and so the method is easy to work. If it is 

 sensitive enough it will be useful in measuring the electric absorption 

 of condensers in terms of resistance. 

 It has the advantage of being practically independent of the current 



period for ^ as it should be. 

 



For comparison of capacities the same simplification does not occur. 

 Indeed the method is of very little value in this case, being sur- 

 passed by 16. 



Method 16. 

 (A) [R,R"-R l ,R'-\[W+r' + r"] + W[R l r"-r f RJ = 



t _ 



L, r C" ~ R,, + R tl ( W+ r'r + ") 



The first equation is satisfied by adjusting the Wheatstone bridge so 

 as to make 



(R I R'R II R)=Q R/'-R l /=Q R l (R ll + r")-R ll (K + r')=Q 

 That is 



R, -R' -^ 



R tl ~ ~R" ~ r" 



We can then adjust W with alternating currents. This is a very 

 good method and easy of application but requires many resistances of 

 known ratio. Many of these, however, may be equal without disad- 

 vantage. A well known case is given by making r' and r" = 0. 



(B) By placing self inductions or condensers in R, and r" instead 

 of the above we have the following 



or VL ,-" or L > - << 



L '' r '-" 



Wr 



+ 1 or - or + VL 1"= 

 FUP c" 



") (Rfi'-RuR)* W(R/'-R ll r f ) 

 W+R" 



Making R" = we have 



c" r " L , 



or - VLp" or -' = 



In case we adjust the bridge to R,W R'R /I = and a condenser 



