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BELL SYSTEM TECHNICAL JOURNAL 



ments, and we will therefore classify them in this way, although in 

 general they have a considerably wider sphere of usefulness than 

 indicated. 



Inductance. A simple shielded bridge for the measurement of in- 

 ductance and resistance has been described by one of the authors ^ and is 

 shown in schematic form in Fig. 4. It comprises two equal resistance 

 ratio arms, an adjustable standard of self-inductance, an adjustable re- 

 sistance standard, a thermocouple milliammeter, two reversing switches, 

 two transformers, and two air condensers. This apparatus is grouped 

 into three separate units, as shown in Fig. 5, one comprising the 



Fig. 5 — Shielded impedance bridge and standards, connected to vacuum tube 

 oscillator and heterodyne detector 



standards of inductance, one the resistance standard, and one the 

 remaining parts of the circuit. Each of these units is shielded electro- 

 statically. The last assembly constitutes the balance element of the 

 system, by means of which the unknown and standard impedances 

 are compared. This unit may be used alone for the comparison of 

 two impedances of any type since the only condition for balance is the 

 exact equality of impedances in the two arms. Using in addition the 

 standard inductance and resistance shown, it is adapted particularly 

 for measuring inductance and effective resistance. The inductance 



