ELECTRICAL METHODS 527 



traversed by this mobile electrode, and this footage, at intervals of ten 

 feet, is recorded on the chart simultaneously with the potential and cur- 

 rent values. 



A carrier current telephone system is provided to allow communica- 

 tion between the operator of the tractor and the operator of the record- 

 ing truck. With a potential difference between the electrodes of approxi- 

 mately 2000 volts, an electric current of 1.5 to 5 amperes usually can be 

 maintained. Similar continuous type measurements may be made in 

 submerged areas by using small power-driven cruisers or boats as the 

 moving electrode. 



The mobile electrode has greatly increased the effectiveness of elec- 

 trical methods of prospecting. Usually, when the intermittent type 

 hand-driven stake electrodes are used, three to five hours are required 

 for a series of readings at a station, which consists of measurements made 

 at intervals of every fifty feet along a 5000 foot traverse movement. With 

 the mobile type of electrode these same readings may be made in approxi- 

 mately fifteen minutes. The mobility of the moving electrode has been 

 found to be very good, even in areas of extremely "tough going." The 

 mobile electrode usually requires two operators, one for driving the 

 tractor and maintaining telephonic communication and the other for 

 handling the reel and measuring equipment. 



The continuous contact electrode is employed most conveniently in 

 areas where the surface conductivity is fairly high and the surface is not 

 too rocky. In areas where the surface is rocky, the contact will be 

 irregular. 



Semi-Continuous Electrode System 



A semi-continuous electrode system is illustrated in Figure 319. This 

 system has operating characteristics intermediate between the intermittent 

 stake electrode method and the continuous contact mobile electrode method. 

 The reel contains approximately 10,000 feet of wire and is mounted on a 

 shaft supported by two large steel wheels. These wheels have a diameter 

 of approximately 30 inches and are similar to those used on concrete 

 "buggies." A small hand crank, connected by means of a sprocket chain 

 drive, is employed for reeling-in the wire at the completion of a run. As 

 will be seen from the diagram of connections, the stationary end of the 

 reel is connected to a commutator. The commutator has a single brush 

 contact connected to a transformer. The other terminal of the trans- 

 former is connected to the two insulated handles for contacting the 

 electrodes. The secondary of the transformer is connected to a double- 

 pole, double-throw, key switch. In one position, the switch closes a 

 buzzer circuit which is employed for signalling the instrument truck. 

 In the reverse position, the switch closes a telephone circuit which includes 

 a hand-set for voice communication. 



Three electrodes, or sets of electrodes, A, B, and C, are placed along 



