540 



EXPLORATION GEOPHYSICS 



maintaining the proper grid voltages on the tubes. In the first stage, the grid voltage 

 is maintained by adjustment of i?i to give a predetermined voltage, read on Vi. The 

 variable resistor R2 is used to adjust the total plate current of T2 (read on milliammeter 

 A2) to its proper value with no A.C. signal input. The 

 rheostat Rz is then adjusted to bring the milliammeter Ax 

 to a zero position so that this meter can be used to read the 

 changes in plate current of the tube T2, which are caused 

 by the A.C. signal input. By proper calibration the meter 

 Ai can be made to read the volts (or millivolts) input to 

 the voltmeter directly. 



Low Frequency Alternating Current Meth- 

 ods. — The use of very low frequency alternating 

 current (1 to 10 cycles per second) greatly mini- 

 mizes the mutual inductance between the current 

 and potential circuits and the phase shift effects 

 which occur when the higher frequencies are used. 

 The ordinary low frequency alternator is not prac- 

 tical for field use because of its large size. 



Ambronnf utilizes harmonic current variations 

 from a rotating resistor to produce the desired low 

 frequency alternating current. The frequency de- 

 pends entirely upon the rotational speed of the 

 resistor and may have any desired value. 

 A low frequency harmonic power method is illustrated in Figure 330. 

 The power supply, which may be any suitable D.C. source, such as storage 

 batteries or an engine-driven generator, is connected across a commutator. 

 The commutator comprises 100 or more segments which are connected to 

 a closed ring of resistors. A rotating arm carries a contact brush at each 



Fig. 329. — Vacuum tube 

 A.C. voltmeter. 



Mnergizing 



Fig. 330. — Electrical circuit for low frequency harmonic power method. 



t Richard Ambronn, "Improvements in Process of and Devices for Electrically Exploring the 

 Ground by Means of Alternating Currents of Very Low Frequency," British Patent 308,256, issued 

 June 11. 1930. 



