750 



ELECTRICAL METHODS 



[Chap. 10 



dium on the left, the P.D.R. (in terms of Y bIY a), is greater than 1; if the 

 poor conductor is on the left, the ratio is less than 1. 



For vertical veins the calculation of theoretical P.D.R. 's can be made 

 in three ways. In the first, the assumption is made that the vein is very 

 wide and that for the left side of the vein the effect shown in Fig. 10-79a 

 can be compounded with that shown in Fig. 10-79&. This procedure is 



>?</! 



Poor 



Fig. 10-79. Potential-drop-ratio curves for contacts and vertical 

 dikes (after Zuschlag). 



permissible only when the vein is so wide that the effect of the electrifica- 

 tion of one side on the other side can be neglected. The more rigorous 

 procedure is to figure with a reflection of the source not only on the bound- 

 ary close to the power electrode but also on that away from the electrode. 

 In this manner the curves shown in Fig. 10-79c and d were obtained. 

 They show the effect of a poorly conductive vein in a medium of good con- 

 ductivity as well as the effect of a vein of good conductivity imbedded in a 



