ELECTRICAL METHODS 533 



operator to give the next point of reading before the reelman leaves, and 

 for the reehiian to give his location to the operator before a reading is 

 made at the new point. Abnormal readings or trends in the recorded values 

 should be noted and checked by additional field measurements, if neces- 

 sary. Careful field work is a prerequisite to proper interpretation. The 

 instrument operator, or preferably a computer, should calculate and plot 

 the values obtained for each point as the field work progresses. In this 

 manner abnormal readings can be detected and checked without undue 

 delay. Although the various steps appear somewhat involved, the opera- 

 tions may be conducted in a rapid manner. 



Instruments and Methods. — In the majority of the conductive 

 electrical methods, the quantity measured is the potential created at the 

 surface of the ground for some particular electrode arrangement. Ob- 

 viously, a great many different types of measurement may be made ; hence, 

 a large ramification of methods, apparatuses, and field procedures exists. 

 Limited space prevents a full treatment of the possible field procedures. 



The methods discussed below are described because of their commercial 

 use and because they are illustrative of certain general principles of 

 operation. The descriptions are not intended to cover all possible modi- 

 fications of any of the methods. Instead, an attempt has been made to 

 incorporate in the description of the different methods, various rami- 

 fications of instrumental technique. Thus, in the simple volt-ammeter 

 method which is described first, a potentiometer employing a null-point 

 type of measurement is described. It is obvious that other accurate 

 methods of measuring potentials could be employed. 



Mi///^fffm^m'l 



S^lf^ry 



^n 



-<D — 



■ Pttttnft»*^tttr I 



Fig. 324. — Apparatus for simple volt-ammeter resistivity 

 measurements. 



Simple Volt- Ammeter Method 



A schematic diagram of the electrical connections is shown in Figure 

 324. Power is usually furnished by a bank of 22y^ volt, heavy duty "B" 

 batteries, such as used for radio plate supply. For average areas the 

 potential need seldom be greater than 180 volts, while in dry desert areas 

 potentials of 300 to 400 volts are often necessary. The batteries should 



