MUTUAL IMPEDANCES OF GROUND-RETURN CIRCUITS 641 



the calculated and observed values. However, for the best agreement 

 it is found necessary to assume a different earth conductivity at 500 

 cycles than at 60 cycles. Thus, while at 60 cycles the indicated earth 

 conductivity is 4.2 X 10~^^ abmhos per cm. cube, at 500 cycles it is 



0.2 



O 

 O 



O 



a. 



ill 



0. 



(O 



2 

 I 

 O 



0.1 

 0.0 7 



0,05 

 004 



003 

 0.02 



0.01 



0.007 



0.00b 

 0.004 



0.003 

 0.002 



0.001 



0O00 7 



00005 

 _ 00004 



D 00003 



I- 

 D 



2 0.0002 



UJ 



o 

 z 

 < 



o 

 < 



a. 

 o 

 z 

 < 



o 



z 

 < 



I- 



u} 

 If) 



UJ 



cr 



0.0001 



10 



20 30 



50 



100 200 300 500 1000 2000 3000 5000 



HORIZONTAL SEPARATION IN FEET 



10,000 20,000 



Fig. 12 — Resistance and reactance of ground-return circuits. Frequency 60 cycles. 



2.76 X 10~^^ However, a computed curve for 500 cycles, using a 

 conductivity of 4.2 X 10~^^ falls below the experimental curve by only 

 30 per cent. Table I gives the values of earth conductivity required to 



TABLE I. 



Cross Keys Tests. 



Earth Conductivity Giving Best Agreement Between Calculated and Measured Values of 



Mutual Impedance. 



Indicated earth conductivity 

 from Carson's formulas 



