A SHIELDED BRIDGE FOR INDUCTIVE IMPEDANCE 



147 



due to the use of long distributing wires, encased in grounded conduit, 

 for supplying the testing current. C3 may consist chiefly of the ground 

 capacitance of the outer layer of the detector coil winding and d that 

 of dead-end coils of the reference standard, Zs. 



1. c; 





Fig. 6 — Bridge circuit with stray admittances 



Some of the currents flowing along the paths provided by these 

 capacitances will complete their circuits external to the bridge network 

 proper and will not affect the balance; for example, that through 

 capacitances Ci and d in series. Other currents, however, will flow 

 unsymmetrically through parts of the bridge circuit; for instance, 

 that through Ci and C3 in series and the arm Zx; also, that through 

 C2 and Ci in series, returning through the ratio arm Ri. These latter 

 currents and others of the same sort affect the potential distribution 

 of the bridge and hence the values of the impedances required for 

 balance. Certain of these capacitance currents in the bridge network 

 tend to neutralize or balance the effects of others; for example, that 

 through the arm Zx due to the series action of capacitances Ci and Cz 

 has a balancing effect with respect to that through Ci and C4 and the 

 arm Zs and would be without reaction on the bridge balance if capaci- 

 tances C3 and Ci were exactly symmetrical with respect to the two 

 detector terminals. Such balancing, however, is accidental in nature, 

 seldom satisfactorily complete and, in part, not constant. Even 

 were it made approximately complete for a particular arrangement, 

 the substitution of another detector or the use of another source of 

 testing current would probably destroy the balance. Variable effects 

 would always be present; for example, those due to the changing 

 position of the operator relative to the parts of the circuit or the 



