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BELL SYSTEM TECHNICAL JOURNAL 



If the impedance can be connected across one arm of the bridge and 

 its value is desired with one terminal grounded, the circuit shown is 

 satisfactory. However, these are special conditions, and where the 

 impedance to be measured forms only part of the total series impedance 

 of an arm, or where the potential requirements are different, such as 

 the requirement that the coil be measured with its terminals at equal 

 potential to ground, the bridge shielding becomes a more serious 

 problem. 



In general, the question of selecting the most suitable system of 

 electrostatic shielding for a specific test circuit, resolves itself into a 

 determination of the most advantageous location of the admittances 

 which, as described above, have been arranged to terminate at certain 

 terminals or junction points. The facts which need to be taken into 



Fig. 4 — Completely Shielded Bridge Network. 



consideration are usually so varied that no general rules can be 

 established. A few typical examples in which shielding is applied 

 with considerable success will, therefore, be taken and the selection 

 of suitable shielding for these circuits discussed. 



Examples of Electrostatic Shielding 



Adjustable Resistor 



An adjustable resistor usually takes the form of a dial box in which 

 there are from one to six dials arranged in series in decade formation. 

 Each decade considered by itself is no more difficult to shield than a 

 single resistor. The admittance of the shield, however, has a different 

 effect at each step, which means that the phase angle varies with the 



