636 



BELL SYSTEM TECHNICAL JOURNAL 



cycles. These results have been replotted on Fig. 8, and it is apparent 

 that for separations within the range of 20 to 500 ft. the mutual resist- 

 ance increases rapidly in almost linear relation to the frequency. For 

 the frequency range and circuit lengths involved in this series of tests, 

 it would appear that a formula for the mutual resistance, based on a 

 d.-c. distribution in the earth is inadequate. 



0.07 



0.06 



I- 

 111 



HI 



"- 0.05 



O 



o 

 >n 



a. 



UJ 



Q. 



10 0.04 



X 



o 



ai 



z 0.03 



< 



if) 



cr 



<0.02 



I- 



D 



2 



0.01 



200 



400 600 



FREQUENCY 



8 00 



1000 



Fig. 8 — Cross Keys tests. Variation in mutual resistance with frequency and 



separation. 



The mutual-inductance curve of Fig. 9 has been computed according 

 to the formulas given by Campbell, and for comparison purposes the 

 mutual inductances derived from the mutual reactances shown on Fig. 

 3 are also plotted. It will be seen that the observed mutual induc- 

 tances decrease as the frequency is increased, and that while the trend 

 of the observed values is towards agreement with the calculated values 



