SHIELDING IN HIGH-FREQUENCY MEASUREMENTS 



571 



condition of one terminal at ground potential. Thus D is shown 

 trapped to the ground shield. For this case, the shielding may be 

 simplified considerably. Fig. 8 shows the mechanical construction of 

 the combined resistance and capacitance standard used with the 

 bridge unit for these measurements. The unit is shown with the top 

 of the outer shield removed. The capacitance, in accordance with 

 the shielding diagram, is double shielded, while the resistance requires 

 only a ground shield. 



Another bridge of the comparison type but using capacitance ratio 

 arms is described by Kupfmuller "^ with a complete description of the 

 shielding involved. 



Fig. 8 — Shielded Resonance Unit, Top Panel Removed. 



A bridge of the general comparison type having self-contained 

 standards of resistance and capacitance is shown with top panel 

 removed in Fig. 9. It may be used for a wide variety of measurements 

 such as series or shunt resonance, and direct comparison, by using the 

 switches controlled by the small dials on the extreme left and right to 

 throw the standards into various combinations. 



Another bridge circuit which is interesting from the shielding point 

 of view is the Owen bridge.^ This is a skew bridge in which the ratio 

 arms are 90° out of phase instead of being equal. For this reason 

 any admittance introduced in one arm by the shielding cannot be 

 compensated for by any equal admittance in another arm. Two 



■* Von K. Kupfmuller, " Uber eine Technische Hochfrequenz Messbrucke," Elek. 

 Nach. Tech., September, 1925, pp. 263-270. 



* D. Owen, "A Bridge for the Measurement of Self Inductance," Proc. Phys. Soc. 

 London, October, 1914. 



