A Shielded Bridge for Inductive Impedance Measure- 

 ments at Speech and Carrier Frequencies ' 



By W. J. SHACKELTON 



Synopsis: A shielded, a-c, inductance bridge adapted to the measure- 

 ment of inductive impedances at frequencies up to 50,000 cycles is described. 

 The bridge comprises a balancing unit and associated standards of induc- 

 tance and resistance. The balancing unit has resistance ratio arms specially 

 constructed to meet the requirements imposed by the above frequency 

 range. The reference standard makes use of inductance coils of a new type, 

 their cores being of magnetic instead of non-magnetic material as is usually 

 the case. The use of such cores results in coils that are smaller and hence 

 better adapted to assembly in a multiple shielded standard. 



The bridge is completely shielded so as to eliminate, to a high degree, 

 errors due to parasitic capacitance currents. The shielding is also arranged 

 so as to permit the correct measurement of either "grounded" or "balanced- 

 to-ground" impedances. A series of diagrams is shown for the purpose of 

 indicating the function of each part of the shielding system. 



Equations expressing the errors resulting from any small residual capaci- 

 tance unbalances in the resultant bridge network are given and calculations 

 made of the balances required for the desired degree of measurement 

 precision. Test data are presented illustrating a method of experimentally 

 checking the residual shunt and series balances from which it is concluded 

 that the bridge is capable of comparing two equal inductive impedances 

 of large phase angle with an accuracy at the maximum frequency of 0.02 

 per cent for inductance and 1.0 per cent for resistance. 



Introduction 



THE limitations of the ordinary unshielded bridge network as a 

 means of making precise a-c. measurements at speech frequencies 

 were early recognized by telephone engineers. The solution of cross- 

 talk problems arising in connection with the use of cable circuits was 

 found to require an exact knowledge of the capacitive balances existing 

 between such circuits at speech frequencies. For the ready and accu- 

 rate determination of the capacitances defining these balances, to- 

 gether with their associated conductance values, G. A. Campbell 

 devised the "shielded balance." ^ This is a bridge network having 

 its parts individually and collectively shielded so as to define exactly 

 the mutual electrostatic reaction of each with respect to all other parts 

 of the electrical system affecting the balance condition. 



As a means of more completely treating the cross-talk problems of 

 cable circuits, Campbell conceived also the very valuable idea of 

 "direct capacity" as distinguished from the "ground" and "mutual 

 capacities" in use up to that time.-^ The shielded balance was found 



1 Presented at the New York Regional Meeting of the A. I. E. E., New York, 

 N. Y., Nov. 11-12, 1926. 



2 G. A. Campbell: "The Shielded Balance," Electrical World and Engineer, 

 April 2, 1904, p. 647. 



3 G. A. Campbell: "Measurement of Direct Capacities," Bell System Technical 



Journal, July, 1922, p. 18. 



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