ELECTRICAL METHODS 523 



desired point of measurement as shown at A in Figure 315. When two 

 electrodes are employed, it is advisable to place them as shown at B. The 

 individual electrodes should be separated a distance approximately equal 

 to the depth to which they are driven into the ground ; as previously 

 mentioned, this depth will vary with the local conditions. If three electrodes 

 are necessary to give the desired current flow, the measuring point 

 should be approximately at the center of an equilateral triangle formed 

 by the electrodes, as shown at C. Other electrode arrangements are 

 illustrated at D and E. In each case an attempt is made to have the 

 electrodes approximate a large extended circular electrode, because a cir- 

 cular arrangement of electrodes produces a minimum distortion of the 

 current lines for a given voltage drop. 



Multi-point electrodes are connected electrically by means of "jumpers" 

 made from short pieces of the same wire used for field lines. It is con- 

 venient to use jumpers which are made of lengths of wire approximately 

 three feet long and provided with clips at each end and at the center. A 

 jumper of this type will connect three electrodes. The contact clips are 

 the conventional heavy-duty battery clips, which are employed for storage 

 battery charging. A clip which has strong spring-actuated jaws is 

 preferable. 



The electrodes are driven into the ground with heavy hammers and are 

 removed from the ground upon completion of a given set of measurements 

 by a special type of stake-puller, or by hammering the side of the electrode 

 to loosen it. 



The freld work is greatly speeded up if two full sets of stakes are em- 

 ployed for each moving electrode. Two electrode men are stationed along 

 the line of movement of the moving electrode. While one set of electrodes 

 is being utilized in the measurement, one electrode man will move and 

 drive in his set of electrodes at the station to be read next. With an ar- 

 rangement of this type, the speed of operation in the field depends upon the 

 spacing between electrodes and the energy of the instrument man. Even in 

 areas of fairly rough terrain, electrode moves may be made in time in- 

 tervals of two to five minutes. 



Power-Driven Electrodes. — For the type of electrical measurement 

 where considerable current must be passed into the ground, the electrodes 

 should have a length sufficient to penetrate the dry near-surface layer of 

 soil. The surface layer during the dry season generally contains less than 

 one per cent moisture. When the per cent of moisture versus depth is 

 plotted, a sharp increase in the moisture content usually occurs at a certain 

 depth, varying from a few inches to 10 feet. In most areas this depth will 

 correspond fairly closely to the depth at which the diurnal temperature 

 changes are small, usually from two to four feet below the surface. The 

 metal electrodes should penetrate into this zone of higher moisture content 

 because its electrical conductivity may be many hundreds of times greater 

 than that of the immediate surface layer. (See resistivity-moisture rela- 

 tionships, as shown in Figure 264.) 



