MUTUAL IMPEDANCES OF GROUND-RETURN CIRCUITS 649 



error due to proximity to the end of the disturbing circuit, and the sec- 

 tion from pole 5614 to 5682, the agreement between measured and cal- 

 culated values is statisfactory. 



TABLE III. 

 AIasstllox Tests. 



Measured Mutual Impedances of Power and Telephone Circuits and Comparison with 

 Values Calculated from Coupling Curves of Fig. 18. 



D.-C. Determination of Earth Conductivity. In considering the ex- 

 perimental results described above, particularly the reasonably good 

 agreement between the experimental coupling curves and those calcu- 

 lated by means of the Carson formulas with suitably chosen earth 

 conductivity, it seemed desirable to investigate whether an experi- 

 mental value of earth conductivity alone would be sufficient informa- 

 tion for the determination of coupling curves with enough accuracy for 

 many purposes. \A'ith this in mind, an investigation has been under- 

 taken of more direct methods for determining the conductivity of the 

 earth or more generally of methods for determining the structure of the 

 earth in a particular location (whether homogeneous or stratified and, 

 if the latter, the thickness of the strata) and the earth conductivity. 

 The work is at present only in an early stage, but a brief statement of 

 the method followed and of the results so far obtained may be of 

 interest. 



The procedure followed amounts to an investigation of the mutual 

 resistance of a number of suitably located ground-return circuits, with 

 direct current. It will be recalled that in an earlier part of the paper 

 it was stated that for a homogeneous earth the mutual resistance of 

 two ground-return circuits, for direct current, can be easily derived, and 

 expressed in a formula involving only the earth conductivity and the 

 distances between the grounding electrodes. For a stratified earth, 

 similar formulas have been worked out involving the distances between 



