758 ELECTRICAL METHODS [Chap. 10 



Since the time constant depends on electrode separation and resistivity, 

 methods have been proposed to measure the time variation in the ground 

 circuit, either with only two electrodes^' or between two potential electrodes 

 in the Wenner-Gish-Rooney or similar arrangements/" 



In most Eltran arrangements used at present, the transients are meas- 

 ured in a separate potential circuit. It has also been found that the 

 Wenner-Gish-Rooney setup is not so suitable for Eltran work because of 

 the large voltages that may be induced by the current circuit into the 

 potential circuit. Therefore, most arrangements now provide for observa- 

 tions outside the current basis in a potential circuit of about 1000 feet 

 electrode separation. The interval between adjacent electrodes of the 

 current and potential circuits varies between one thousand and several 

 thousand feet. 



Various techniques have been used to record electrical transients. They 

 are: (1) direct oscillographic recording, (2) neutralization of transients by 

 two opposing generating circuits, (3) compensation of the transient by a 

 reference signal furnished bj' the power generator or by a locally syn- 

 chronized generator, and (4) controlled alteration of the received transient 

 to give a predetermined (saw-tooth) wave form. 



Direct oscillographic recording was probably first used by Karcher and 

 McDermott. ^ These investigators employed in the primary circuit two 

 electrodes about a half mile apart, supplied through a switch from a 

 storage battery of several hundred volts with currents of 10-20 amperes. 

 Measurements were made half to three-quarters of a mile away in the 

 extension of the primary electrode basis with two nonpolarizable electrodes 

 about 0.1 to 0.2 mile apart, connected to a calibrated D.C. amplifier and 

 oscillograph. The time constant was represented in the form of an 

 "inductance function" which, according to the foregoing, is the apparent 

 inductance due to the skin effect and therefore chiefly dependent on elec- 

 trode base length and resistivity. Hence, the similarity in Karcher's 

 "inductance" and "resistivitj^-slope-function" curves at the same locality. 



In a method described by White, ^^ power is supplied by a 60-cycle 

 generator through a rectifier capable of delivering several thousand volts 

 on open circuit. The rectifier charges a bank of condensers totaling 10 

 to 15 microfa"rads in capacity. The condensors are discharged by a fast 

 mechanical switch which is released by a radio signal received from a 

 transmitter synchronized with the sweep of the cathode-ray oscillograph 

 on the receiving end. In this method the records are evaluated by taking 



«9 L. W. Blau and L. Statham, U. S. Patent No. 2,079,103. 



■"> L. W. Blau, U.S. Patent No. 1,911,137. 



'1 J. C. Karcher and E. McDermott, A.A.P.G. Bull., 19(1), 64-77 (Jan., 1935). 



" G. White, A.I.M.E. Tech. Publ. No. 1216, 4 (Feb., 1940). 



