48 Hl.l.l. SYSTEM TECIIMCAl. JOLRNAL 



funiur would be transferred to the output, i.e., the power ratio Ps/Pr 

 would be infinity. However, this ideal condition cannot be produced 

 in practise so that there is always a finite power loss between the 

 input and (iut|)ut of the transformer which can be measured approxi- 

 mately by the yain of an amplifier calibrated in TU. An internal 



BtUnung 





curt, under Tect 



M«»«ur«rn«ni» of lmpedinc« Balance Between Ltnet and Networks 

 (?ICircu't Teett on Teleptnne Bopeeiert and Carrier) 



rig. I.I^Diagram Showing Principles of 21 Circuit Balance Tests 



path for currents which may produce "singing" or a sustained tone is 

 established if the gain of the amplifier Pr'P,. is greater than the loss 

 P, Pt through the three-winding transformer. As unbalances between 

 network and line become greater the loss through the three-winding 

 transformer becomes less thereby ief|uiring less gain in the amplifier 

 to prcxluce a "singing" coiuliiion. It should be noted in this con- 

 nection that to produce the condition described above exactly, the 

 current received around the "singing" path must be in phase with 

 the starting current. In practise this condition obtains sufficienth 

 accurately so that the gain of the amplifier required to produce 

 "singing" gives an approximate measure of the ini|ie(larue balance 

 lietweeii line ami network. 



1. .Mi-AsiKiiMKNrs ()!• Kksistasci:, R i:.ur.\N( k and Imim-dancf. 



Diagram (a) of Fig. 14 shows the wheatstone bridge circuit for d-c. 

 resistance measurements. It is unnecessary to describe the well 

 known principles of this bridge but mention is made of it here in view 

 of its im|K>rtance and use in telephone maintenance work. It supplies 

 an indis|X'nsiible nuthnd of measurement for certain trouble locations, 

 such as crosses and grounds anfl emlxxlies the fundamental principles 

 uf all null tests. 



Diagram (b) of l-'ig. II gives a bridge circuit for measuring imped- 

 ance, the particul.ir .irrangement shown being for measurements of 

 inip.il-iii. .•>. h.iving inductive reactance. The bridge measurements 



