A Method of Measuring Earth Resistivity

A knowledge of earth resistivity ^ (or specific resistance) maybe of value in determining something of the composition of earth, such for example, as moisture content, whether it contains oil, ore of high conductivity, etc., or in the calculation or mitigation of damages to pipe systems by electrolysis caused by the rettim current of street-railway systems. For some of these or other reasons we may wish to determine the resistivity of limited portions of. the earth. For those cases in which we desire the resistivity of a fairly large portion of earth, extending to a considerable depth, or where there are reasons why we should not disturb the portion of earth to be measured, the following method is suggested: Four holes are made in the earth approximately uniformly spaced in a straight line. The diameter of the holes is not more than lo per cent of the distance between them, and all extend to approximately the same depth, which is usually that at which we are most concerned with the resistivity. In each hole is placed an electrode, which makes electrical contact with the earth only near the bottom, as shown in Fig. i .