104 BELL SYSTEM TECHNICAL JOURNAL 



is very difficult and its solution has been found in only a few special 

 cases. It is true that once we know the electric currents in the screen, 

 we can determine the field on both sides of the screen ; but there is no 

 simple way of calculating these currents exactly. Frequently it is 

 assumed that, in so far as the side opposite to the source is concerned, 

 a perfectly conducting screen is equivalent to a perfectly absorbing 

 screen of the same geometric character. This is equivalent to a 



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

 shown in heavy lines. 





b\ 



Fig. 66 — A source 5 in front of a screen the cross-section of which is 

 shown in heavy hnes. 



hypothesis that the electric current density of the screen is deter- 

 mined by the magnetic intensity impressed directly by the source S. 

 We could take the results obtained from this hypothesis as a first 

 approximation to the true results. The tangential component of the 

 electric intensity calculated on the basis of this hypothesis does not 

 vanish on the screen which means that we have violated the original 

 hypothesis that the screen is a perfect conductor. If the discrepancy 

 is not too great we might look for an additional electric current 

 distribution to reduce this discrepancy. 



