ELECTRIC WAVE FILTERS 



275 



difficulty, as the filters for the type "C" carrier system are assembled 

 in copper boxes or shields as shown in F'ig. 9 and are completely 

 soldered all around, including the lid. This type of assembly was 

 made necessary by the requirements on cross-talk between filters 

 mounted next to each other on a vertical rack. The cross-talk between 

 two filters mounted side by side was required to be 135 db below the 

 signal currents, or, in other words, the ratio of the desired signal to 

 the cross- talk had to be a little over 5,000,000 : 1. The chief cause of 



ELEMENTS TO 

 BE RESONATED 



R-VARIABLE 

 RESISTANCE 



DETECTOR 

 Fig. 8 — Simplified schematic of L-C bridge. 



such cross-talk between two pieces of apparatus is electromagnetic 

 coupling between them ; to reduce this coupling eddy-current shielding 

 is emplo3'ed. Eddy-current shielding is accomplished by surrounding 

 the source of a var>qng electromagnetic field — a coil, say, or the coils 

 in a filter — by some metal, such as copper, brass or aluminum, which 

 has good conductivity. 



To make clear the effect of a shield on an inclosed inductance, a 

 brief discussion of shielding may be of interest. If a coil through which 



