SHIELDING IN HIGH-FREQUENCY MEASUREMENTS 567 



setting of the dial. If the admittance to the shield is small, this 

 effect will not be very great and in any case it is always the same for 

 a given setting and hence may be included in a calibration. 



In shielding several decades in series, admittances between decades 

 are introduced. Effects due to these admittances can be taken care 

 of completely by the use of nested shields "as already shown in Fig. 1/. 

 For a resistance box of five or six dials, this type of shielding becomes 



Fig. 5 — Six-Dial Shielded Adjustable Resistor. 



prohibitive from a size and cost standpoint and in consequence such 

 shielding is usually not attempted. The use of a single shield for all 

 decades of a resistor means that the impedance of two or more dial 

 settings is not exactly equal to the sum of the impedances of each 

 setting by itself. If the difference is appreciable the only alternative 

 to the expensive type of shielding mentioned above is the use of a 

 calibrated value for every combination of dial settings. This error 

 in additions is smaller the lower the resistance, and usually may be 

 neglected for values below 100 ohms. 



