ELECTRICAL METHODS 



459 



of 3 feet. They are pointed at the ground end and provided with an 

 insulated handle for the operator. 



Power Sources. — Portable gasoline-engine driven alternators are com- 

 monly used as a power source. The alternator should develop from 250 

 to 1000 watts output at a frequency well up in the audio range, preferably 

 from 100 to 500 cycles per second. The load voltage output of the alter- 

 nator should have a value of 100 to 200, or a suitable output transformer 

 should be employed to allow proper impedance matching between the 

 alternator and the load. 



Amplifiers. — A two- or three - stage transformer - coupled, audio- 

 frequency amplifier may be used. It should be well shielded electromag- 

 netically and electrostatically. The amplifier should be rugged, portable 

 and non-microphonic. Plate supply may be small "B" batteries, and "A" 

 batteries may be used to heat the filaments of dry-cell tubes. The design 

 of the amplifier follows that of conventional audio-frequency equipment. 



Headphones. — The headphones should be a light-weight, rugged type 

 and should be provided with rubber cushions to minimize extraneous 

 interfering noise, such as that due to the wind. 



A.C, Methods Based on Comparison of In-Phase Current 

 Components. — Equipotentials are obtained in a manner analogous to 

 that used in the D.C. method. The points of equipotential are traced out on 

 the surface of the ground by moving one probing electrode until a min- 

 imum of sound is detected in the headphones.* 



The A.C. Potential Ratio Method. — The principle of the A.C. poten- 

 tial ratio method developed by Broughton Edget may be illustrated by 

 reference to Figure 274. The 

 method consists essentially in em- 

 ploying a bridge circuit which 

 makes contact with the ground at 

 three points A, B, C, the point B 

 being chosen such that AB equals 

 BC. Each of the two ratio arms 

 AO and CO contain a condenser 

 and resistance connected in series. 

 An audio frequency alternating current (about 500 cycles per second) is 

 passed through the ground between distant earthed conductors. This 

 arrangement is used to compare the potential drops Vi and V2 between 



* If the frequency of the alternating current is not sufficiently low, the investiga- 

 tion must take into account the phase relation between the energizing current and the 

 potential field at the surface of the ground. Zuschlag has proposed a method wherein 

 an alternating electric or electromagnetic ground field is created, and measurements 

 are then made to determine the ratio between the electric potentials or the electro- 

 magnetic fields at desired points in the area under investigation. 



t Broughton Edge and Laby, loc. cit., p. 50. 



Fig. 274. 



-Alternating current bridge method, 

 with ratio bridge. 



