784 



ELECTRICAL METHODS 



[Chap. 10 



plane-tuned amplifier with phones. At each station the observer records 

 the number of turns in the horizontal coil required to obtain a balance, 

 holding the axis of the double coil vertical with the plane of the large coil 

 first in a north and south, and then in an east and west direction. Com- 

 pounding of the two readings yields the ratio of the quadrature horizontal 

 to the vertical in-phase field and gives, therefore, approximately the ratio 

 of the axes of the ellipse of polarization. This resultant may be plotted 

 as an arrow whose direction points toward the conductor and whose length 

 is greatest near its edges. Points of equal vector amplitude may be joined 

 by lines of equal intensity of the out-of-phase component. 



MajorAxh 



MinorAxis 



— 1 f^ua&afurtM 



I I— — -AAAAWWWWVW 1 [ 



X 



2C 



(a) (b) 



Fig. 10-106. (a) Double-coil arrangement in reference to polarization ellipse (after 

 Edge and Laby). (b) Bruckshaw's modification of Bieler-Watson system. 



With the simple Bieler-Watson system sharp nulls are often unobtain- 

 able. If the ellipse is not vertical, an in-phase e.m.f. appears in the 

 vertical coil and a quadrature e.m.f. appears in the horizontal coil. Fur- 

 thermore, the phase shift between the vertical and horizontal components 

 is not always 90°. An instrument designed to measure any phase shift 

 between the vertical and horizontal components has been constructed by 

 Bruckshaw^^ (Fig. 10-1066). The circuit is comparable with that of a 

 compensator if one considers the horizontal frame as "reference" coil. An 

 inductance with resistance, as well ls a capacitance with resistance, are 

 connected in two parallel branches to this coil and its (reference) e.m.f. 



92 Loc. cit. 



