A SHIELDED BRIDGE FOR INDUCTIVE IMPEDANCE 



149 



of testing current and connected at C; likewise, one around the 

 detector is connected at D. It will readily be seen that these shields 

 localize the effects of the various capacitance currents. Those circu- 

 lating within the shields have, of course, no effect exterior to the 

 shields, while those flowing between the various shields directly or 

 by way of ground enter and leave the bridge system at definite points. 

 By themselves, these shields do little good but they are necessary in 

 order to make the next step, the balancing of the capacitances, prac- 

 ticable. 



Generally it will not be found convenient to shield the current 

 supply apparatus, especially if this is a power-driven generator. Also, 

 to promote greater flexibility in respect to testing with a wide range 

 of frequencies, it will often be desirable to substitute one source of 

 current for another and likewise one detector for another. The 

 shielding of this apparatus should therefore be reduced to a minimum. 

 This is readily effected by making both the supply and detector 

 branches of the bridge one of the windings of a transformer. This 

 winding can be electrostatically shielded without affecting its trans- 

 former action and then any desirable source of current supply or 

 any type of detector can be magnetically coupled with it.'^ Intro- 

 ducing this change the circuit becomes as shown in Fig. 8. The 



Fig. 8 — Bridge circuit with shielded transformers and local shields 



capacitances of the various shields to ground being still variable, the 

 next step to correct this condition would simply be to add a ground 

 shield around each. At this point, however, it becomes necessary to 

 consider the ground admittance relations of the impedances to be 

 tested. 



6 U. S. Patent No. 792248, June 13, 1905. 



