SHIELDING IN HIGH-FREQUENCY MEASUREMENTS 565 



in Fig. 3. Each of the four impedances constituting the arms may 

 be considered as any combination of individual impedances. With 

 the shields connected as shown, the total admittances are reduced to 

 three ; namely, between B and D and from B and D to ground. These 

 admittances do not affect the bridge balance and, therefore, are not 

 objectionable. However, if we add input and output circuits and 

 follow the same system of shielding, we get the result shown in Fig. 4. 

 In this case it is impossible to concentrate all of the admittances at 

 B and D. Neglecting for the present the ground at D, we have 

 added variable admittances from A to B, to D and to ground. The 

 only way of overcoming this difficulty is to use double shielding as 







Fig. 3 — ^Bridge Network Using Shielded Impedances. 



shown, adding an outer shield to the impedance across AC and con- 

 necting it to D. This puts a fixed admittance across AD, but as we 

 have not made any distinction between the four arms of the bridge, 

 this admittance may generally be placed across an arm where it can 

 be taken care of satisfactorily. If in addition we ground D, the 

 admittances reduce to a single one from B to ground. 



Admittance to Ground of Unknown Impedance 



From the above it would appear that the general bridge circuit is 

 susceptible of a simple complete solution, since the shielding shown 

 in Fig. 4 is equally applicable to all cases. This would be true if the 

 unknown impedance to be measured in the circuit had no admittance 

 to ground. This is usually not the case. We generally have an 

 additional requirement that the potential condition with respect to 

 ground of the impedance during the measurement be defined in 

 some way. 



