ELECTRICAL MEASUREMENT BY ALTERNATING CURRENTS 311 



ards of equal as well as of different values, as the mutual inductances 

 can have any ratio to each other. 



For twisted wire coils r t = r' very nearly. See method 23 for the 

 use of the resistance, A. 



Method 25. 



In Fig. 6 remove the shunt R' and self induction L. 



This method then depends upon the measurement of the angular 

 deflection when a self induction or a capacity is put in the circuit of 

 the small coil of the electrodynamometer and comparing this with the 

 deflection, when the circuit only contains resistance. 



The resistance of the circuit, r, is supposed to be so great compared 

 with R that the current in the main circuit remains practically un- 

 altered during the change. 



There is also an error due to the mutual induction of the electro- 

 dynamometer coils which vanishes when r is great. 



'Z i r+R" 

 L-j-- -grr-J 



These formulas assume that the deflection is proportional to 6. This 

 assumption can be obviated by adjusting 6 = 6' when we have 



1 



W R" 



These can be further simplified by making R " R". 

 The method thus becomes very easy to apply and capable of con- 

 siderable accuracy. As the absolute determination depends on the 

 current period, however, no great accuracy can be expected for absolute 

 values except where this period is known and constant, a condition 

 almost impossible to be obtained. The comparison of condensers or of 

 inductances is, however, independent of the period and can be carried 

 out, however variable the period, by means of a key to make the change 

 instantaneously. 



Method 26. 



Similar results can be obtained by putting the condenser or induc- 

 tance in R" instead of r, but the current through the electrodynamo- 

 meter suspension is usually too great in this case unless r is enormous. 

 We have in this case for equal deflections, 



1 //r 7?" _ v 7?"\ 



^ or PL'" = R" (R"+r) p r >'' 

 where r, and R" are the resistances without condenser or self induction. 



