1000 THE BELL SYSTEM TECHNICAL JOURNAL, SEPTEMBER 1952 



and is balanced by the calibrated capacitor cs and resistance standard rs. 

 The ranf2;e capacitor cr consists of several mica capacitors for extending 

 the range of cs, as will be described below ; and zs is merely a compensat- 

 ing impedance, essentially an inductive two-ohm resistor. The circuit 

 is thus l)asically ([uite simple and avoids the use of switches or other 

 complications which would impair performance at these high freciuencies. 



Capacitors cp and cs are worm-driven air capacitors with a range of 

 about 220 mm/, and were specially designed for this bridge. In the case of 

 cs, any direct conductance between rotor and stator would result in an 

 effective series resistance which would vary both with freciuency and 

 capacitor setting, and therefore re(|uire laborious correction. This was 

 avoided by arranging the construction so that the rotor and stator are 

 mounted on independent insulating supports to the ground panel, thereby 

 completely eliminating direct conductance from rotor to stator. While 

 this results in some conductance from test terminal xl to ground, the 

 amount is small and its effect is negligible because of the relatively low 

 impedance values measured. In the case of cp, on the other hand, it is 

 important to minimize series resistance and inductance to avoid con- 

 ductance and capacitance corrections which would change both with 

 frecjuency and capacitor setting. This was accomplished by careful design 

 of the rotor brush using silver contact surfaces and center-fed connections 

 to both rotor and stator. 



The conductance standard, gp, and resistance standard, rs, were de- 

 signed to emphasize high-freciuency performance. Deposited carbon resis- 

 tors^* on ceramic rods |" in diameter and |" long mounted on small 

 decade rotors were used, so arranged that only one resistor on a rotor is 

 in the circuit at any time, and that adjacent resistors are short-circuited 

 by means of auxiliary shorting brushes to eliminate shunting admittance 

 which might vary with frecjuency. For gp the resistance values are such 

 that the two lower decades and the slide-wire rheostat each have a 

 residual conductance of 333 micromhos, thereby avoiding the use of 

 resistors exceeding 3,000 ohms in value which would be more likely to 

 vary with frequency. The structure is designed to minimize series in- 

 ductance and to maintain constant capacitance for all settings. For rs, 

 on the other hand, it is necessary to maintain constant inductance for 

 all settings. This was accomplished by adding small wire-loop compen- 

 sating inductors in series with individual resistors in the 10-ohm and 

 100-ohm decades when necessary. To minimize the over-all inductance, 

 the resistor rotors are placed very close together and are dri^Tn l\v gear- 

 ing from the corresponding dials. 



The range capacitor, cr, has already been mentioned. It consists of a 



