IMPEDANCE BRIDGES FOR MEGACYCLE RANGE 



1005 



The successful use of a center-lapped 1 raiisfoinier for latio anns in a 

 465-KC direct capacitance hiidge ' iiulicaied that the icsistanct! i-alio 

 arms r1, r2 of Fig. I might be omitted if a .suital)l(> transformer could i)c 

 developed for higher freciuencie.s. The traiisfoiiner group of the T>a])<)ra- 

 tories succeeded in producing a transformer with a deviation from unity 

 ratio of less than 0.1 per cent over a fr(H|uency range from 0.5 to 20 

 megacycles. This was made possilile l)y precise location of the windings 

 in tine milled grooves in the form of re\(>rsed helices, cut on a longitudin- 

 ally-split brass cylinder for the inner winding, and on a surrounding phenol 



OSCILLATOR 



Fig. 5 — Schematic of the 20-megacycle general-purpose bridge showing both 

 the series (impedance) and parallel (admittance) bridge circuits combined in a 

 single unit. 



fibre cylinder for the bifilar outer winding which serves as the bridge 

 ratio arms. Electrostatic shielding limits the direct capacitance be- 

 tween primary and secondary to less than 0.01 /i/x/. The core material is 

 compressed powdered molybdenum permalloy. This transformer was 

 the nucleus around which the general purpose bridge was built, and the 

 resulting bridge is shown schematically in Fig. 5. 



In Fig. 5, the letters a, b, c and d designate the four bridge corners, 

 and T is the ratio-arm transformer already described. Apparatus to be 

 measured by the admittance method is connected to terminals c and d, 

 and is balanced by the calibrated capacitor cp and conductance standard 

 GP. To use the series reactance method, cp and gp are set at minimum 

 settings, apparatus to be measured is connected to terminals xl and x2, 



