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BELL SYSTEM TECHNICAL JOURNAL 



necessary only to definitely ground one of the bridge terminals to 

 which the impedance is connected and establish the proper initial 

 capacitance balance of the bridge for this condition. This is readily 

 done by grounding junction point C and adjusting the capacitance 

 from 5 to C to equal the ground capacitance of D. The shielding 

 system may remain the same as in Fig. 10. 



Fig. 10 — Shielded bridge circuit with balancing condensers 



In the case of the bridge being described it was desired to have a 

 means of verifying by reversal the degree of balance of the ratio arms 

 and also that of the impedance arms. The bridge is therefore equipped 

 with reversing switches for this purpose. Due to the appreciable 

 effect produced by a relatively small capacitance unbalance arising 

 from factors present only when the arms are in circuit, it is quite 

 important to be able to do this when a high degree of accuracy is 

 desired. To effect the proper reversal, however, certain conditions 

 must be definitely maintained. In reversing the impedance arms 

 none of the inherent bridge admittances should be disturbed ; that is, 

 only the unknown impedance and the standard as read should be 

 transferred. On the other hand, in reversing the ratio arms not only 

 should the resistance element of these arms be transferred but also all 

 associated shunt admittances. Moreover, in transferring these ad- 

 mittances they must be absolutely unchanged. A further require- 

 ment is that the ratio arm reversal must not occasion the shifting of 

 any capacitances shunting the impedance arms. To accomplish these 

 objects a suitable arrangement of shielded switches was worked out 

 and added to the circuit of Fig. 10, the result being as shown in Fig. 11. 

 In this arrangement all capacitances between the various parts of the 

 switches which are subject to change due to physical movement of 



