ELECTRICAL METHODS 



585 



Therefore, 



Hy = ^ — \ {I + cos 6) do \ cosAdA 



or 



X 



(3) 



Equation 3 specifies the field produced at the surface of the earth by 

 current flowing from a point source into homogeneous, non-magnetic 

 earth.* 



The field at P due to current flowing into a sink located on the x axis 



at a distance L from the source is 7 • . This field has the same direc- 



L — X 



tion as the field produced by current leaving the source. Hence, the total 



magnetic field at the point P is : 



X L — X \ X L — X / 



(4) 



In this equation the value of H is given in gauss provided / is expressed 

 in electromagnetic units (absolute amperes) and x and L in centimeters. 



d • FRACTION OF TOTAL DISTANCE = \ 



Fig. 365. — Plot of— versus fraction of the total distance (—) along a traverse 

 between electrodes. 



Equation 4 specifies the magnetic field at the surface due to current 

 flow in homogeneous, non-magnetic earth. (This result does not take 

 into account anomalous surface effects, magnetic effects produced by the 

 return conductor connecting the current electrodes, or the natural mag- 

 netic field of the earth.) 



Figure 365 is a plot of Equation 4 in which the value of H per unit 



* It is of some interest to point out that the same result would be obtained if the 

 current flowed in a straight line vertically downward from the current source. 



