446 EXPLORATION GEOPHYSICS 



higher, in order to minimize the effects of the contact resistance between 

 the probe electrodes and the earth. 



A number of commercial galvanometers are available for this work. 

 It is preferable to employ a rugged pivotless instrument of the pointer type 

 with the coil suspended between taut bronze ribbons. The suspension type 

 instruments must be properly leveled while readings are made. 



Electrodes. — The simplest ground contact would consist of a metal 

 rod driven into the earth. This type of contact is unsuitable because of 

 electrochemical effects. Due to the dissolved salts present in the ground, the 

 moist earth acts as an electrolyte and a potential difference is created 

 between the metal electrode and the earth. The magnitude of the electro- 

 chemical potentials depends on the metal constituting the electrodes, the 

 concentration of the electrolyte, and the temperature of the electrodes. 

 For instance, temperature differences between electrodes may create poten- 

 tials of considerable magnitude when one electrode is in the hot sun and the 

 other electrode in the shade. 



Non-Polarizing Electrodes 

 One type of electrode whose potential is not appreciably affected by 

 the chemical properties of the soil consists of a metallic electrode immersed 

 in a supersaturated solution of one of its metallic salts, the solution being 

 contained in a semi-permeable or porous cup that is in contact with the 

 earth. This type of electrode is described in detail on p. 520. 



Testing Electrodes in the Field. — During field work, the non-polar- 

 izing electrodes should be tested occasionally by immersing both of them 

 in a single earthenware or glass container and observing the difference 

 in potential with the aid of a galvanometer or a potentiometer. This dif- 

 ference in potential should be less than one millivolt if chemically pure 

 materials have been used. While not in use, the electrodes should be kept 

 immersed in a non-metallic jar (glass or glazed earthen ware) containing 

 the same strength solution as in the porous cups. Distilled water should be 

 used for mixing all solutions, if possible. In field work it may be difficult to 

 obtain pure water, and should it be necessary to use water containing 

 appreciable quantities of dissolved minerals, care must be taken to keep 

 the solutions in both electrodes the same. This may be accomplished by 

 emptying both cells into a large container and refilling with the mixed 

 solution once or twice a day. Any difference in potential between the two 

 cells must be compensated by adding or subtracting from the field reading. 

 The compensation may be positive or negative depending upon the relative 

 polarity of the electrodes and the field readings. 



Field Procedure for Locating Equipotential Lines 



In surveying an area, an arbitrary starting point, say station 1, is 

 selected. The meter and tripod are set up near this station and connected 

 by a short length of wire to the non-polarizing electrode buried at the sta- 



