A SHIELDED BRIDGE FOR INDUCTIVE IMPEDANCE 



151 



at ground potential. If, however, the testing current is appHed at 

 the points B and D, the equipotential points are the junctions A and 

 C, the sum of whose ground admittances would then be made equal 



Fig. 9 — Shielded bridge circuit showing location of ground admittances 



to that of D and the arm CD again balanced with respect to ground 

 potential. In this case there must be no ground admittance from 

 junction B. To permit of testing under both conditions, point A 

 and all connected conductors are protected with a shield which is then 

 connected to the point C. Point B is likewise enclosed by a shield 

 connected to point D. These two main shields then represent the 

 junction points C and D of the bridge and are fixed with respect to 

 capacitance to ground by a ground shield which may be common to 

 the two. 



There now exist, external to the local shields, direct capacitances 

 only between points A and C and between B and D (which do not 

 affect the bridge balance), and from points C and D to ground. These 

 latter do, of course, affect the balance. Two courses are open. Their 

 effective resultant value shunting the arm CD can be determined and 

 allowed for by calculation. Such calculations would involve a con- 

 siderable amount of labor, however, and can be avoided very simply by 

 providing in the opposite arm an exactly equal shunt capacitance. 

 To permit adjusting the ground capacitances of points C and D, an 

 adjustable condenser is connected to ground from the point having the 

 lower value. With the apparatus connected as shown this is usually 

 point D. The shielded system then becomes as shown in Fig. 10. 



When impedances, which in actual service are grounded at one 

 terminal, are to be tested, the matter is much simpler. Then it is 



