Chap. 10] ELECTRICAL METHODS 671 



in this particular procedure) and one electrode are placed at a point on the 

 reconnaissance traverse at which the gradient is steep and regular. The 

 assistant then takes the second electrode, connected to the potentiometer 

 or milliammeter by a 50- to 100-foot length of insulated wire and applies 

 it to the ground in a succession of trial points situated approximately on 

 the circumference of a circle, the radius of which is determined by the 

 length of the wire. The operator, in watching the direction and the 

 amount of deflection of the galvanometer or milliammeter for each of the 

 trial contacts, will be able to direct the assistant to the point of equal 

 potential at which no deflection is observed. A stake is driven at this 

 point and the operator moves up to it while the assistant moves forward 

 to locate the third equipotential point. With a type of electrode to which 

 a small milliammeter is permanently fastened, the survey may be expe- 

 dited by leaving one electrode stationary with the assistant while the 

 operator moves the other to a point of zero current. Equipotential-line 

 surveys should be closed back to the starting point. As soon as an equi- 

 potential line has been traced, the disposition of the electric field and the 

 approximate location of a negative center will be known. Its precise 

 position is estabhshed by surveying other equipotential lines at arbitrary 

 mtervals of constant potential difference which may be spaced with the 

 potentiometer. 



With a miUiammeter, potential differences may be determined as follows: 

 Two resistors are placed in series with the electrodes, one of unknown 

 resistance Ro (resistance of milliammeter included) and the other of variable 

 resistance R. First, a reading /i is taken with only Rq in the circuit, so 

 that 7i = (Va — Vb)/Ro , where Va and Vb are the electrode potentials. 

 Then R is inserted and adjusted until one-half of the former reading is 



obtained, so that I2 = -^ — j — ~ . Hence, Rq = R and, therefore, the po- 



tential difference Va — Vb = Rh ■ 



C. Interpretation 



The spontaneous polarization method is one of the few electrical methods 

 in which the depth of penetration cannot be controlled. Hence, any 

 quantitative interpretation must necessarily be of an indirect nature. 

 Anomaly curves may be calculated for bodies of simple geometric shape; 

 depth and dip can then be determined approximately from them by inter- 

 polation. Such calculations may be readily made for spheres with vertical 

 or inclined polarization and for bars and sheets. These calculations give 



^' If the galvanometer of the potentiometer is used, the slide wire dial must be 

 set. at zero. 



