A SHIELDED BRIDGE FOR INDUCTIVE IMPEDANCE 143 



to be especially adapted to the precise measurement of direct capacities 

 employing the substitution method devised by E. H. Colpitts."* 

 Shortly thereafter, with the advent of loading for telephone lines, the 

 same principles of shielding were extended to apply to bridge networks 

 specially arranged for the measurement of the speech-frequency in- 

 ductance and effective resistance of loading coils. As the successful 

 commercial application of loading required the manufacture of these 

 coils in large numbers to precise requirements, it was quite essential 

 that testing means be available permitting a relatively unskilled tester 

 to determine quickly whether the proper adjustment of the coils had 

 been made. For this purpose the shielded balance has proved to be 

 extremely valuable. More recently the employment of frequencies 

 up to 50,000 cycles for carrier telephone and telegraph purposes has 

 led to the need for correspondingly precise measurements at these 

 higher frequencies. In this field the advantages of the shielded bridge 

 are so great as to make it almost indispensable. 



While the fundamental principles of the shielded balance are essen- 

 tially the same for all impedance measurements, the practical applica- 

 tion of shielding to any concrete bridge problem may vary according 

 to the kind and range of impedances to be tested, the frequency 

 range to be covered, and the precision required. It also presents 

 special problems in the design and construction of several of the circuit 

 elements. This paper describes a particular form of shielded bridge 

 which has been developed to meet the conditions commonly en- 

 countered in the measurement of inductance at speech and carrier 

 frequencies. The facts leading to the detailed construction are dis- 

 cussed and some experimental data given to illustrate the performance 

 of the bridge. 



General Features 



A simple schematic diagram of the bridge circuit is shown in Fig. 1 . 

 To avoid confusion, no shielding is shown in this diagram. As will 

 be noted, there are provided two equal non-inductive resistance ratio 

 arms, an adjustable standard of self-inductance, an adjustable resist- 

 ance standard, a thermocouple milliammeter, two transformers and 

 two adjustable air condensers. Physically, this apparatus is grouped 

 into three separate units, one comprising the standards of inductance, 

 one the resistance standard, and the third, the remaining parts of the 

 circuit. The last assembly constitutes what may be considered the 

 balance element of the system, by means of which the unknown and 

 standard impedances are compared. Figs. 2 and 3 show the arrange- 



■• .See Note 3. 



