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BELL SYSTEM TECHNICAL JOURNAL 



effects of parallel loads on the supply generator. The distribution 

 and value of the ratio arm ground capacitances described above are 

 functions of the bridge surroundings; hence they are also subject to 

 change if the bridge is moved from place to place. In the bridge 

 being described, however, the shielding used affords a means of defi- 

 nitely fixing and controlling the various inter-circuit capacitances. 

 Consequently, such variations cannot take place, balances between the 

 resultant capacitance currents can be made as desired, and the bridge 

 measurements are satisfactorily precise. 



Shielding System Used 



It is felt that the merits of the particular shielding system adopted 

 for this bridge can best be brought out by showing, step by step, 

 the reasons for using each of its elements. 



The first step is to simplify, for further treatment, the initial 

 residual capacitance network of the unshielded circuit. This is done 

 by providing individual shields for each part of the circuit that it is 

 desired to have function as an independent unit. Such shields can 

 be connected to one of the terminals of the part enclosed and thus 

 there is substituted, from the standpoint of terminal-to-terminal 

 characteristics, a definite and invariable condition in place of that 

 which was previously a function of the relation of the part to its sur- 

 roundings. For example, as shown in Fig. 7, shields would be placed 



Fig. 7— Bridge circuit with local shields 



around the resistance coils forming the ratio arms Ri and Ri and 

 connected to the junction point A of the system, one enclosing the 

 elements of the standard impedance Zs and another around the source 



