392 ELECTRICAL MEASUREMENTS 



galvanometer is about 1,800 cycles per second, a limit is set 

 above which the telpehone must be used. The sensitivity of a 

 telephone detector may be greatly increased by having it tuned 

 to the frequency of the supply, especially if that is near the fre- 

 quency at which the ear is most sensitive (800 to 1,000 cycles 

 per second). Of course with any tuned detector, the periodicity 

 of the current must be kept constant. 



Capacity Measurements. If two perfect condensers are to 

 be compared, they may be placed in the arms P and X (compare 

 with Fig. 225). The arms M and N may be non-inductive 

 resistances. In this case 



Zi M = KM 

 Z N = R N 



7 i 



Substituting in (26) gives 



RN R 



RN 



In practice the comparison of ordinary condensers is not so 

 simple, for an energy loss may occur in one or both of them. As 

 the behavior of a condenser with an imperfect dielectric depends 

 on the frequency, it is important that the correct periodicity be 

 employed. 



If energy losses be present, the phase of the current in R M 

 will probably not be the same as that in R N and no adjustment 

 of these resistances can be found which will cause a zero indi- 

 cation of the detector. If a telephone be used, there will be a 

 considerable range of adjustment over which the sound is faint 

 but never entirely disappears. 



If an energy loss be introduced into the arm of the bridge 

 having the smaller power factor, the currents in R M and R N may 

 be brought into phase and an exact balance obtained. Wien 

 accomplishes this by the use of a series resistance in the arm 

 containing the better condenser. 15 



