SHIELDING IN HIGH-FREQUENCY MEASUREMENTS 573 



compensated for it may cause considerable error, particularly in the 

 measurement of high impedances at high frequencies. For this reason 

 it is desirable that all shields be supported by insulating material of the 

 highest quality such as hard rubber, glass or quartz and that only the 

 minimum amount necessary for satisfactory mechanical support be 

 used. 



The wiring it will be noticed in Fig. 9 is shielded by brass tubing. 

 This shielding is insulated from the conductor by means of bushings, 

 only enough being used to insure that the conductor and shield do 

 not change their relative positions with respect to each other. The 

 insulating bushings used most generally are either hard rubber or 

 glass beads. 



Even after taking these precautions it has been found necessary 

 for the highest precision work at the highest frequencies, to introduce 

 a conductance compensator in the form of a small adjustable con- 

 denser in which the dielectric is an insulating material such as phenol 

 fiber. By this means the amount of conductance in one arm may be 

 varied to obtain correct compensation. The balance, once obtained, 

 does not vary appreciably with frequency. Such an adjustment is 

 used with the bridge shown in Fig. 9. 



In bridge input and output transformers, which must generally be 

 double-shielded, the shielding is rendered more difficult due to the 

 requirement of a low conductance between shields. This demands a 

 much more expensive construction than the simple requirement of 

 complete electrostatic shielding. 



Limitations of Shielding 



Having discussed the uses and advantages of shielding, it may not 

 be amiss to discuss briefly some of the limitations. As already brought 

 out, the introduction of shielding always brings with it some additional 

 admittance. Since this admittance is a function of frequency it is 

 natural that shielding should introduce more trouble, the higher the 

 frequency. However, it is also equally true that the stray admittances 

 due to lack of shielding introduce more trouble, as the frequency is 

 increased. 



In general, it may be said that if shielding a circuit is found to have 

 a definite advantage at moderately high frequencies, it will have an 

 advantage up to the maximum frequency at which the circuit is used. 

 The principles outlined already apply over the whole range of com- 

 munication frequencies. Where shielding is found to result in fre- 

 quency limitations, it is due to the added admittance introduced with 

 it and not due to inherent defects in the principles involved. 



