596 



EXPLORATION GEOPHYSICS 



I dE 



Evidently, rr -77 is a maximum for 6 equal to 90°. That is, the direction of 



II do 



the resultant field is determined most accurately by rotating the coil until 

 its plane is parallel to the magnetic field. 



The 

 by the 



Figure 8 Curve 



magnitude of the induced voltage is often shown graphically 

 so-called figure 8 curve. (Figure 372.) f This curve shows the 



relative value of the induced voltage 

 in the loop as a function of the angle 

 that the loop makes with the magnetic 

 field for the case that the magnetic 

 field is horizontal. When the coil is 

 in the position AA^ perpendicular to 

 the direction of the field, the magnetic 

 flux through the coil and consequently 

 the induced voltage is a maximum. 

 As the coil is rotated further, the flux 

 through the coil decreases until at 

 position DD^ the coil is parallel to the 

 field and the induced voltage is a mini- 

 mum. If the rotation of the coil is 

 continued the direction of the flux 

 through the coil will reverse, thereby 

 producing a reversal of phase of the 

 induced voltage. 



Fig. 372. — Plan view of a direction-finding 

 coil in a uniform field. "Figure-eight curve." 

 (Proceedings, Institute of Radio Engineers.) 



Field Apparatus. t — The complete 



direction-finding apparatus consists of 



a direction-finding coil, mounting head, 



tripod, amplifier (for audio-frequency range) or detector and amplifier 



set (for higher frequencies), and a pair of headphones or a vacuum tube 



voltmeter. 



One type of direction-finding apparatus is illustrated in Figure 373. 

 The mounting head on which the direction-finding coil is pivoted is pro- 

 vided with a sighting arrangement (similar to gun peep-sights) whereby 

 the axis of rotation of the coil may be aligned quickly with the center of 

 the energizing coil. A graduated vertical arc is attached to the pivoted 

 plate holding the direction-finding coil. The vertical-angle index mark 

 and level-bubble arc are attached to the movable arm, which is adjusted 

 by a thumbscrew. 



The head rotates on a vertical axis and the azimuth angle may be read 

 on a graduated scale. The entire assembly is mounted on a ball-and-socket 



t J- J. Jakosky, "Electrical Prospecting," Proc. Institute of Radio Engineers, Vol. 16, No. 10, 

 Oct. 1928. 



t J. J. Jakosky, "Operating Principles of Inductive Geophysical Processes," A.I.M.E. Geo- 

 physical Prospecting, 1929, pp. 138-176. 



