ELECTRICAL METHODS 



597 



plate. The direction-finding coil is electrically connected to the vacuum- 

 tube set-box which contains a detector (so arranged that it functions as 

 an audio-frequency amplifying stage when using low-frequency coils) 

 and a two-stage tuned transformer-coupled audio-frequency amplifier. 

 The entire apparatus, consisting of heterodyne control dial, controls and 

 batteries, is contained in a waterproof aluminum box 4^ by 4;^^ by 17 



Fig. 373. — Electromagnetic direction-finding apparatus 

 for high frequencies. A, mounting head; B, alignment 

 sights; C, graduated arc for reading vertical angle; EF, 

 vertical rotation axis; G, azimuth scale; H, ball and socket 

 plate; /, battery compartment; J, voltmeter; K, head- 

 phones. (A.I.M.E. Geophysical Prospecting, 1929.) 



inches. A double-range voltmeter is placed behind a waterproof "port- 

 hole" by means of which filament and plate voltages may be read. The 

 control knobs are placed on a recessed panel for protection against mech- 

 anical injury. Conventional type headphones are provided so that the 

 operator can detect the point of minimum signal strength when deter- 

 mining the direction of the resultant field. The same mounting head and 

 5et-box are used for both low and high frequency work. 



