454 



BELL SYSTEM TECHNICAL JOURNAL 



between certain of the impedances which enable us to evaluate one of 

 them in terms of the others. Thus the bridge is essentially a method 

 of comparing" impedances. The impedances of the bridge may consist 

 of resistance, capacitance, self and mutual inductance, in any combina- 

 tions, and they may actually form a much more complicated network 

 than the simple circuit shown in Fig. 1. Consequently, the number of 



Fig. 1 — Schematic of the impedance bridge reduced to its simplest form. 



different bridges which can be devised for the measurement of im- 

 pedances is extremely large. However, since only four junction points 

 are significant, any bridge circuit may be reduced to a network of six 

 impedances connected between these four points, as shown in Fig. 1. 

 These impedances are direct impedances, that is, there are no mutual 

 impedances between them- 



If a potential is applied at BD and the balance condition is that 

 the potential be zero across AC, then the points BD are called the 

 input or power source terminals and the points A C are called the output 

 or detector terminals. The impedances Zbd and Zac then act simply 

 as shunts across the power source and detector respectively and do not 

 affect the balance relation. The balance is not affected if the power 

 source and detector are interchanged in a bridge reduced to this simple 

 form and hereafter no distinction will be made in this respect. 



After the bridge has been reduced to the form of Fig. 1, the equation 

 for balance is 



ZcdZab = ZbcZadi 

 from which 



ZbcZad 



ZcD = 



(1) 



