544 



EXPLORATION GEOPHYSICS 



& :M. 



-t_J>i 



Fig. 332. 



-Circuit diagram for vacuum tube ratio 

 instrument. 



RATIO INSTRUMENT 



A very convenient vacuum tube instrument t for determining the ratio E/I 

 possesses excellent flexibility, sensitivity, and ease of operation. Use is made of the 

 linear relationship between the applied plate potential and the amplitude of the 



X current in an oscillating vacuum 

 tube circuit. In the usual geophys- 

 ical application of the instrument, 

 variations in the energizing current 

 produce variations in the potential 

 applied to the plate of an oscillating 

 triode tube. These variations in 

 plate potential cause corresponding 

 variations in the oscillating current. 

 The oscillating current is rectified 

 and flows through a calibrated po- 

 tentiometer. The potential drop 

 across the potentiometer varies with 

 the oscillatory current which, in 

 turn, varies with the energizing 

 current. 



A schematic wiring diagram of 

 the instrument is shown in Figure 

 332. The energizing electrodes and 

 the power supply are connected to 

 terminals X and Y which are connected to a heavy wire potentiometer Ri. A multi- 

 point switch vS"2 and resistor R2 provide shunts for higher current ratings. The current 

 through Ri is indicated by the milliammeter h. The vacuum triode V is connected 

 as an oscillator through transformer Ti. The poten- 

 tial applied to the plate of this oscillator is that of 

 the battery B plus the potential drop across a 

 desired part of the potentiometer R\. Transformer 

 Ti must be of the shielded type, with special pre- 

 cautions taken to prevent leakage between the 

 energizing current circuit which is connected to 

 windings 1 and 2 and the potential circuit which 

 is connected to winding 3. 



Winding 3 of the transformer is connected to 

 a full wave rectifier (of the copper-oxide type), 

 and the rectified output passes through Rs. The 

 drop across Rs furnishes the potential for balancing 

 against the E.M.F. existing between the two poten- 

 tial electrodes which are connected to the terminals 

 X' and Y' of the diagram. The potentiometer 7?b, 

 with its small battery and reversing switch S3, is 

 used: (1) to neutralize the natural ground poten- 

 tials and the potential drop across Rs due to 

 minimum plate current flow of the oscillator and 

 (2) to compensate for the drift (usually negligible) 

 of the oscillator output. The sensitivity of the 

 galvanometer G is increased by depressing the 

 closed circuit switch Kt. which removes the shunt R 



Fig. 333. — Vacuum tube ratio in- 

 strument. X and Y, current ter- 

 minals; X' and F', potential ter- 

 minals; 5"3, neutralizer reversing 

 switch; £1, filament and plate volt- 

 meter; /i, current meter; G, gal- 

 vanometer; S2, range switch for I\. 



from the galvanometer circuit. 



t J. J. Jakosky, "Method and Apparatus for Electrical Exploration of the Subsurface," U. S. 

 Patents 2,162,086; 2,162,087, issued June 13, 1939; "Method and Apparatus for Electrical Explora- 

 tion of the Subsurface," Canadian Patent 374,475, issued June 14, 1938. 



