Chap. 10] 



ELECTRICAL METHODS 



783 



ture field is caused by currents induced in subsurface conductors. The 

 field depends on frequency, radius of the primary coil, current strength, 

 and the apparent conductivity of the subsurface section. It is measured 

 by a compensator coil which is connected to the primary coil through a 

 ''reference" transformer and mounted in the same level (h) with the de- 

 tector coil, while the effect of the in-phase (primary) field is compensated 

 by a neutralizing coil laid out on the ground. The detector coil is con- 

 nected to an amplifier with a vacuum-tube voltmeter circuit. 



A receiving arrangement involving the determination of (horizontal) 

 out-of-phase components in terms of (vertical) in-phase fields is known as 

 the Bieler-Watson method. It makes use of a dual coil receiver, con- 

 sisting of a large rectangular frame rigidly connected to another smaller 



Primari/ Coil ; h=0 



Meier 



PidcupCoil 



Fig. 10-105. Compensator and primary loop arrangement in Koenigsberger 

 ring induction method. 



one at right angles to it. The purpose of the small frame is to pick up 

 the field corresponding to the major axis of the ellipse of polarization 

 (almost vertical, due to the in-phase loop field), while the large frame will 

 pick up the field in the direction of the minor axis of the ellipse (Fig. 

 10-106a). This field is usually horizontal, and is in quadrature with the 

 loop field. The e.m.f.'s induced in the coils are pulled back into phase by 

 a condenser across the vertical coil and connected in opposition to a de- 

 tector. The number of turns in the horizontal coil is changed until 

 balance is obtained. The detector is a three-stage, transformer coupled. 



91 Edge and Laby, op. cit., pp. 64-67, 283-286. H. G. I. Watson, Canad. Geol. Sur- 

 vey Mem. 165, 144-151. J. McG. Bruckshaw, Phys. Soc. Proc., 46, 350 (1934). 



