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BELL SYSTEM TECHNICAL JOURNAL 



ment of the parts in this unit. Fig. 4 illustrates the appearance of the 

 standard inductance unit and Fig. 5 shows how the units are associated 

 when a test is being made. The thermocouple milliammeter indicates 

 the total effective test current applied to the bridge and forms a 



Fig. 1 — Schematic diagram of bridge circuit 



means of determining when this current has been adjusted to the 

 desired value. 



In operation, the air condensers are first adjusted to produce an 

 initial or zero balance of the residual electrostatic capacitances of the 

 apparatus. Aside from the initial balancing, the operation of the 

 bridge follows the usual practise; that is, the standards of inductance 



Ficr. 2 — Balance element of shielded bridge. Rear view of pane! removed from case 



and resistance are alternately adjusted until the balance detector 

 indicates a condition of zero potential difference at every instant 

 between the bridge points to which it is connected. The inductance 

 and resistance values as indicated in the standard arm are then equal 

 (within the precision limits of the bridge) to the corresponding con- 

 stants of the unknown impedance. 



