538 EXPLORATION GEOPHYSICS 



shaft as the power commutator is used for reversing the potential periodi- 

 cally. This commutator changes the alternating potential created by the 

 commutated energizing current to a pulsating uni-directional potential which 

 can be measured by a standard direct-current potentiometer. 



The double commutator system eliminates the variable ground currents 

 from the measurements, because any difference in a uni-directional poten- 

 tial existing between the two inner electrodes is automatically nullified due 

 to the rapid reversals. 



The commutator may be either hand-cranked or motor-driven, A motor- 

 driven commutator maintains constant speed which permits more accurate 

 work and also eliminates the services of an assistant. Between the two 

 commutators is a guard or slip-ring which is grounded as shown in the 

 wiring diagram in order to prevent leakage from the current commutator 

 to the potential commutator.* 



Precautions must be taken to prevent stray or leakage currents from 

 influencing the potential readings. In addition, the lines and reels connected 

 to the instrument should be laid in a manner which will minimize induction 

 effects. Although direct current is secured from the power batteries, the 

 "make" and "break" of this direct current, while being changed into 

 a periodically reversed current, creates bad transient effects. These 

 are usually minimized by having the power circuit "make" a short time 

 interval before, and "break" a short time interval after the potential circuit. 

 When this procedure is used, steady-state conditions exist during the time 

 interval in which measurements are made. 



The previously described constant potential system may be incorporated 

 in the commutator method, with a resultant increase in general overall 

 accuracy and speed of reading due to the fact that variations in ground 

 potentials are removed by the commutator. 



Photographic Method for Minimizing Effects of Ground Currents. 



— The current can be passed into the ground at predetermined time in- 

 tervals, and the reoccurring pattern may be viewed with any of the con- 

 ventional type oscillographic equipment. For field use, the cathode ray 

 oscilloscope has been found particularly suitable. For this use an oscillo- 

 scope, with suitable amplifigation, is incorporated into the resistivity meas- 

 uring instrument. The ground potentials are fed into an amplifier and then 

 impressed across one pair of deflection plates. The energizing current is 

 passed through deflection coils that have their axes at right angles to the 

 potential deflection plates. The slope of the path of the resultant pattern 

 depends upon the relative magnitudes of potential and current, and the 

 instrumental constants. 



Due to the persistence of vision, accurate measurements of slope may 

 be made with pulses arriving as slow as one per second. 



By use of suitable panchromatic film and a wide aperature (fast) lens, 



* Constructional details for this equipment are available at the Carnegie Institution 

 of Terrestrial Magnetism, Washington, D. C. 



