JOURNAL 



OF THE 



WASHINGTON ACADEMY OF SCIENCES 



Vol. V OCTOBER 4, 1915 No. 16 



PHYSICS. — A method for yneasuring Earth resistivity. Frank 

 Wenner, Bureau of Standards. 



A knowledge of earth resistivity may be of value in determin- 

 ing something of its composition, such for example as moisture 

 content, whether or not it contains oil or ore of high conductivity, 

 etc., or in the calculation of damages to pipe systems by the 

 return 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 the measurement should be made without 

 disturbing the portion 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 10 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. Two of these electrodes serve 

 as current terminals and two as potential terminals in the meas- 

 urement of the resistance. 



Knowing the resistance, the depth of the holes and the dis- 

 tance between them we have data from which the effective 

 resistivity in the vicinity can be calculated. 



561 



