562 wenner: earth resistivity 



In case a is the distance between the holes, h is the depth of 

 the holes, p is the resistivity, and R the measured resistance, then 



4iTraR ^iraR ..s 



p = = (1; 



^ 2a _ 2a n 



where n varies between 1 and 2 according to the ratio of the 

 depth of the electrodes to their distance apart. If the holes are 

 not in a straight line or are not of a uniform depth or spacing 

 the resistivity is easily calculated from the depth of each of the 

 holes and the distance of each from each of the other three. 



Concerning the resistance measurements there is in general 

 no need for a high accuracy. There is therefore no reason why 

 we may not use an ammeter for measuring the current and a 

 voltmeter for measuring the resulting difference in potential 

 between the potential terminals, providing no current is drawn 

 from these terminals. As the voltage to be measured is low and 

 the resistance between electrodes and earth high, errors would be 

 introduced if the ammeter-voltmeter method were used in the 

 ordinary way. 



The following potentiometer arrangement, using alternating 

 current to obviate the more serious difficulties which might arise 

 on account of polarization with direct current, seems to answer 

 the purpose fairly well. The current terminals or electrodes are 

 connected to a source of alternating voltage of suitable value, 

 and across the line is connected a step-down transformer, the 

 low voltage side of which is connected to the ends of a slide wire. 

 One of the potential terminals is connected to one end of the slide 

 wire and the other through a vibration galvanometer to the adjust- 

 able contact on the slide wire. An ammeter is connected into 

 a lead to one of the current terminals and a voltometer across 

 the ends of the slide wire. On account of the polarization at 

 the current electrodes a variable inductance is connected into 

 one end of the leads, for the purpose of bringing the test current 

 in phase with the voltage of the low side of the transformer. 



