ELECTRICAL METHODS 



521 



not change with the passage of a moderate amount of current. Electrodes 

 of this type do not completely eliminate electrolytic or contact potential 

 phenomena,! because there must be a contact between the electrolyte seep- 

 ing through the porous cup and the earth. However, contact potential 

 effects between the two electrolytes, the electrolyte or natural moisture 

 in the earth and the electrolyte of the electrode, 

 is very much smaller than the contact potentials 

 between a metal and the earth electrolyte.* 



A non-polarizing electrode of simple design 

 is shown in Figure 314. The hollow bakelite 

 tube functions as a convenient handle and also 

 acts as a reservoir for the saturated copper 

 sulphate (CUSO4) solution. The solution is 

 added by removing the expanding rubber stop- 

 per at the top of the hollow bakelite tube handle. 

 Contact with the solution is made by means of 

 a pure copper bar, %"yi^" in cross section, 

 with the lower end doubled back to give a larger 

 surface area in the solution and hence a lower 

 contact resistance. 



Electrodes of the non-polarizing type have 

 two chief disadvantages in field operation: (a) 

 Precautions must be taken to insure that the 

 materials used are chemically pure and that the 

 materials and electrolyte of one electrode do not 

 differ from those of the other electrode, (b) The electrodes are fragile 

 and cannot be subjected to rough treatment. 



In using non-polarizing electrodes, it is necessary to make a small 

 hole in the ground, usually by means of a small hand or garden trowel. 

 The porous cup portion is placed in the hole. A small quantity of water 

 or dilute salt solution should be poured around the electrode and the earth 

 lightly tamped to give better electrical contact.** 



These operating details make the use of non-polarizing electrodes 

 in field work rather tedious. This is especially marked in those methods 

 wherein the potential electrodes must be moved rapidly from one set-up to 

 another. Attempts have been made, therefore, to eliminate the use of non- 

 polarizing electrodes by appropriate field technique and design of equip- 

 ment. Various types of commutating arrangements have been devised 

 to minimize the effects of spurious potentials at the potential electrodes. 

 These will be described later in con i unction with the commutator methods. 



Fig. 314. — Simple non-polariz- 

 ing electrode. 



a — Expansion stopper and cap 



b — Handle and reservoir 



c — Terminal 



d — Porous cup 



e — Expanding stopper and guard 



chain 

 / — Rubber sleeve 



t Atsushi Matsubara, "Method of Prospecting Underground Ore Bodies," U. S. Patent No. 

 2,153,636, April 11, 1939. 



Glen Peterson, "Method of Geophysical Prospecting," U. S. Patent No. 2,190,324, Feb. 13, 

 1940. 



* The electrolytic or contact potential between two electrolytes is usually negligible 

 in commercial work. 



** The procedure described in this sentence is frequently termed "wetting down." 



