434 



BELL SYSTEM TECHNICAL JOURNAL 



45 



=0 100 500 1000 5000 



FREQUENCY IN CYCLES PER SECOND 



Fig. 12 — Observed effect of covers on cylindrical shields. 



contact between the cover and the cylinder it may be shown that at 

 low frequencies this is immaterial while at higher frequencies the 

 reverse is true. However, even at these higher frequencies a small 

 overlap (as for B) is sufficient. 



Examples of the effect of covers upon the shielding efficiency are 

 also given by Figs. 8 and 11. The effect of covers on a copper cylinder 

 as shown by curves A and B (Fig. 13) is of special interest. 



50 100 200 500 1000 2000 5000 



FREQUENCY IN CYCLES PER SECOND 



Fig. 13 — Observed shielding efficiency of a copper cylinder. 



Use of Copper 



The curves A and B of Fig. 13 give the shielding efficiency vs. 



frequency of a cylinder made of copper. This cylinder has an inside 



diameter of 2 5/8" and is 4.5" high. The thickness of the wall is 



1/16". The shielding efficiency is here entirely due to the eddy 



