A SHIELDED BRIDGE FOR INDUCTIVE IMPEDANCE 



153 



the switch parts are either short-circuited or connected across opposite 

 bridge points and hence do not affect the bridge balance. The small 

 capacitance Cr between the switch shield and that of the ratio coil 

 i?2 shunts this coil and is not carried with it on reversal. For this 

 reason a corresponding capacitance Cr, shunting the coil Ri, is pro- 



Fig. 11 — Complete circuit diagram of balance unit with shielding 



vided and connected to the opposite point of the switch. This is 

 adjusted by test to equal the value of Cr. The diagram of Fig. 11 

 represents completely the circuit and shielding used for the balance 

 unit of the bridge. 



While, from the standpoint of the bridge balance alone, the parts 

 comprising the standard impedance can be shielded with a local and 

 ground shield as shown in Fig. 9, unless the standard has a very limited 

 range, the resulting calibration is exceedingly laborious to make and 

 use. To reduce calibration difificulties, additional shields can be used; 

 this, of course, increasing the cost of construction. In arriving at the 



