ELECTRICAL METHODS 447 



tion. The other electrode, which is connected to the meter by the long 

 length of insulated wire, is contacted with the earth's surface at a point 

 approximately 100 to 200 feet from the first electrode. The polarity and 

 magnitude of the potential difference between the electrodes are noted, and 

 the electrode is then moved to another location. If the reading is lower, 

 the movement has been in the right direction ; the electrode is then moved 

 to various locations until a point is found where no potential difference 

 exists. This point is now marked with a suitable stake and constitutes the 

 second station on the equipotential curve. The galvanometer and its elec- 

 trode (which were at station 1) are now moved to the new location (station 

 2) and the process repeated to find another equipotential station. By con- 

 tinuing this procedure, the equipotential stations will eventually close in 





M \c 



G 



FiG. 207. — Photographic view of equipment for measuring earth poten- 

 tials, a, non-polarizing electrode; b, potentiometer; c, tripod; d, anchor stake; 

 e, reel. 



on the first arbitrarily selected point. This completes one group of equi- 

 potential stations and a line drawn through these points is an equipotential 

 curve. Another arbitrary starting point is selected about 100 feet inside the 

 curve and the steps outlined above are repeated, thereby locating another 

 group of equipotential stations. On continuing this procedure, the 

 enclosures will become smaller in size ; the smallest enclosure, theoretically 

 a point, is the negative center. The various stations are now located on 

 a suitable map, prepared from data obtained by an alidade and plane table 

 survey, and are connected by closed lines so as to form equipotential curves. 

 The various curves will be concentric with the negative center as their 

 origin. 



