SOME EQUIVALENCE THEOREMS OF ELECTROMAGNETICS 103 



the contribution of that portion of the equivalent layer which vv^as 

 removed by the screen. 



s >1< 



Fig. 5« — A source 6" in front of a screen the cross-section of which is 

 shown in heavj' lines. 



n jO/'^^.? § 



5 



Fig. Sb — A source 5 in front of a screen the cross-section of which is 

 shown in heav\' lines. 



The case of a hole in a perfectly absorbing screen (Fig. 6a and Fig. 

 6h) can be treated in the same manner and the reciprocity existing 

 between this and the preceding case is quite evident. In terms of 

 the sources previously defined the field to the right of the screen is 

 — S" ; by (20) this is the same as 5 — S' . 



If the screen is a perfect conductor, the problem is much more 

 complex. The screen will support electric currents but not magnetic 

 currents. The densities of the electric currents are not calculable 

 directly from the field 5 but from the condition that the component of 

 the electric intensity tangential to the screen vanishes. The problem 



