ELECTRICAL METHODS 535 



potentiometer. As soon as the current value has been noted by the operator, 

 he releases the foot switch, thereby breaking the current circuit, and then 

 disconnects the potentiometer. If the galvanometer remains at balance it 

 indicates that the natural ground potential has not varied, and the current 

 and potential values read are the values desired. If an appreciable change in 

 natural ground potentials has occurred, the ground potentials should again 

 be neutralized, and the procedure repeated until check ratios of E/I are 

 obtained. 



Instruments using this principle may readily be made portable and 

 convenient for field use. Figure 325 illustrates a successful form of 

 apparatus. The folding legs bring the panel to a convenient height when 

 the operator sits on a folding camp chair. 



Controlled Potential Method 



At depths of more than a few hundred feet, it is necessary that the 

 electrical measurements have an accuracy greater than can be obtained 

 with the simple volt-ammeter procedure described above. More accurate 

 results can be obtained by bringing the potential (or current) up to some 

 fixed value and measuring the corresponding current (or potential). 

 Since a known potential is applied, the only reading that need be made is 

 the current value. 



In practice, it is customary to use a source of constant potential and 

 vary the energizing current in order to obtain the set potential value.* 



If, during a series of readings, it becomes advisable to change from 

 one fixed value of the potential to another, the effect of the change as a dis- 

 turbing influence may be evaluated by taking double readings for both 

 potentials at one or more positions of the electrodes. The ratio of E/I is 

 obtained by dividing the potential by the instantaneous value of the current 

 required to balance this impressed potential. 



The apparatus shown diagrammatically in Figure 326 has a general 

 similarity to the apparatus for the simple volt-ampere method, and corres- 

 ponding parts will not be described again. The desired potential is regu- 

 lated by means of a multi-point switch ^'i connected to a resistor, across 

 which there is a potential drop of 100 m.v. This potential drop is main- 

 tained by adjusting a resistor Ri to give a predetermined reading on the 

 voltmeter E-^. A closed circuit inish-button key /lo controls the sensitivity 

 of the galvanometer. A closed-circuit, double-acting key K3 normally 

 allows only the neutralizer potentiometer Pi to be connected in the circuit, 

 thus providing the necessary means for neutralizing the natural earth 

 potentials. 



* Maintaining a constant value for the potential is preferable to keeping the cur- 

 rent constant or bringing it to some predetermined value because, in the latter pro- 

 cedure, the current must first be adjusted to its desired value and then the poten- 

 tiometer employed for measuring the potential must be adjusted. During the manipu- 

 lation of the potentiometer the current ordinarily will have changed, partly because 

 of polarization of the batteries supplying the current and partly because of changes 

 in resistance in the earth in the immediate vicinity of the electrodes due to polarization. 



