ELECTRICAL METHODS 589 



367. f The field wires should be No. 14 stranded copper, or larger, and 

 the electrodes should be of the multiple stake type well "wetted down" to 

 minimize losses. 



The area to be studied lies between the two grounded terminals or 

 electrodes. The longer the legs of the "U," that is, the further away the 

 portion of the cable parallel to the line of the electrodes, the less is the 

 efifect of the primary current flowing in the surface cable. The legs of 

 the "U" preferably should be once or twice the depth to be worked, and 

 measurements should not be made nearer the legs of the "U," or the 

 electrodes, than about half the depth. The magnetometer is placed on line 

 with the two energizing electrodes 1 and 2. At this position, the field 

 strength measuring apparatus is then subjected to two artificially created 

 fields : (a) the essentially vertical magnetic field H^ created by the flow 

 of current through the energizing wire a, and (b) the complex field (repre- 

 sented as a single field He) created by the flow of current in the subsurface. 



Fig. 368. — Horizontal component magnetometer: A, torsion head; B, torsion scale; 

 C, level; D, telescope; E, tripod; F, scale, with sky mirror (not shown); G, clamp knot). 

 (Courtesy of Union Oil (Tompany of California.) 



Measuring Apparatus 



The measuring apparatus is designed or oriented to respond primarily 

 to the horizontal component He of the field. 



Magnetometer (Suspension Type). — One type of magnetometer 



t J- J. Jakosky, "Method and Apparatus for Determining Underground Structure," U. S. 

 Patent 1,906,271, issued May 2, 1933. 



