ELECTRICAL METHODS 



539 



a photographic record of the pattern for each electrode position may be 

 obtained. The exposure should be such as to give only a faint trace for 

 each sweep of the beam. By recording a series of sweeps, the true slope 

 is accurately shown by the record. Varying natural ground potentials 

 merely cause a shift in the starting point of the trace, and do not affect 

 the slope of the recorded patterns. 



Alternating Current Methods 



Various alternating current methods are employed for earth resistivity 

 measurements. f As indicated earlier, the characteristic feature of these 

 methods is the use of an alternating current power supply instead of a 



direct or commutated current supply. 



Medium Frequency Alternating Current Method. — Power for ener- 

 gizing the ground for this work is obtained by alternators driven by a 

 small gasoline engine or a D.C. converter which operates on storage battery 

 power. Frequencies in excess of 50 cycles per second are undesirable due 

 to errors introduced by phase shift, etc. 



The output terminals of the alternator are connected to the primary of 

 an input transformer. The secondary of this transformer is tapped in order 

 that the secondary impedance may be matched to the electrode-circuit im- 

 pedance. By means of a multi-point switch and the field rheostat of the 

 alternator, the energizing current may be controlled as desired. The 

 alternating current potentials are measured by means of a vacuum tube 

 voltmeter. The required sensitivity is determined by the electrode con- 

 figuration used and the power available for the current circuit. 



Fig. 328. — Circuit diagram of vacuum tube voltmeter for measuring 

 alternating current potentials. 



Vacuum Tube Voltmeter 



The vacuum tube voltmeter shown in Figures 328 and 329, consists of a detector 

 tube Ti used to rectify the A.C. signal and a stage of direct coupled amplification 

 which amplifies the D.C. output from the first stage. Calibration is dependent upon 



t L. F. Athy and II. R. Prescott, "Method of Electrical Prospecting," U. S. Patent No. 

 2,172,271, Sept. 5, 1939. 



P. W. Klipsch and S. S. West, "Electrical Prospecting with Alternating Current," U. S. Pat- 

 ent No. 2,231,013, Feb. 11, 1941. 



P. W. Klipsch, "Method of Electrical Prospecting," U. S. Patent No. 2,293,024, Aug. 11, 

 1942. 



