A SHIELDED BRIDGE FOR INDUCTIVE IMPEDANCE 



161 



of the bridge circuit. For this purpose the transformer core is made 

 in toroidal or ring form and the windings, both primary and secondary, 

 are uniformly distributed about its circumference. The wound toroid 

 is also completely enclosed in a sheet iron case. 



The winding which is connected to the bridge has an electrostatic 

 shield completely surrounding it for the purpose of concentrating all 



Fig. 13 — Reversing switciies 



capacitance currents at one point. Around this localizing shield there 

 is a second or ground shield. These two shields are made of sheet 

 copper approximately No. 30 gauge (0.010 in.) in thickness. The 

 inner winding terminal leads are brought out through a small brass 

 tube leading into a terminal chamber which is an extension of the 

 localizing shield. Since the admittance between the localizing and 

 ground shields forms a major part of one of the balanced admittances 

 shunting the impedance arms, it is desirable that the capacitance 

 component be of low value and essential that it be constant. The 

 conductance component should be negligibly small. To attain this 

 end, the shields are separated at definite distances by means of hard 

 rubber rings turned to fit the outer corners of the inner shield and the 

 corresponding inner corners of the enclosing shield. These rings are 

 made of the smallest cross-section consistent with mechanical strength 

 requirements so as to introduce the minimum amount of solid material 

 into the space between the shields. This minimizes the capacitance 

 11 



