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wires on the surface. A brief outline of this derivation is given here. 

 We first find the formulae for the components of the electric field due 

 to a current flowing in a straight wire of length 2a parallel to the 

 surface of the earth and at the height // above it, assuming the air 

 to be replaced by a medium of finite resistivity p\. This part of the 



-0.08 



-0.09 

 -0.10 



4 5 6 7 



VALUES OF r' 



Fig. 11 — Real and imaginary parts of iVi<-*(r'). 



derivation follows closely the work involved in the previous case of 

 wires on the surface. We next find the electric field due to a current 

 in a vertical wire extending from the surface of the earth up to the 

 height H in the assumed medium. This part of the derivation is 

 simpler since there is circular symmetry. Upon combining these two 

 results, we obtain the field due to a current flowing through three 

 sides of a rectangular circuit beginning and ending at the surface of 



