Chap. 10] 



ELECTRICAL METHODS 



785 



is split up into its in-phase and quadrature components. The currents in 

 these two branches differ by 90° if L = CR^. To measure the in-phase 

 and quadrature components of the e.m.f. induced in the vertical coil, the 

 latter is connected across two slide-wire resistances as shown in Fig. 10-103. 

 Equal sensitivity for in-phase and quadrature components may be ob- 

 tained by making the currents in both branches equal to each other and 

 db45° different in phase from the main current. This is true when Leo = 

 I/ojC = R. If, further, L^u = R -\- Rz , and if r is the setting of the 

 potentiometer in the upper branch (in-phase with Vi) and s is the setting 

 of the potentiometer in the lower branch (in quadrature with Fi), the 

 condition for balance is given by 



Yi 



72 



1 



2iR + Ri) 



(s + JT). 



(l0-53a) 



Relative determination of electromagnetic fields may be made by meas- 

 uring field ratios and phase differences in successive ground intervals. In 

 other words, the field at one loca- 

 tion serves as a reference for that 

 at an adjacent location. In the 

 application of this procedure to the 

 vertical component, two coils are 

 laid flat on the ground in horizon- 

 tal position or are carried by two 

 surveyors with straps around their 

 waists (see Fig. 10-1076). For 

 horizontal-intensity determinations, 

 two coils are held in a vertical posi- 

 tion ; their direction is kept parallel 

 with the direction of strike if the 

 primary cable has been laid out 

 parallel with the strike. Instru- 

 ments for the relative measurement 

 of intensity ratios and phase differ- 

 ences have been constructed as 

 adaptations of potential ratiometers 

 and compensator bridges. The 

 type shown in Fig. 10-107a is an 



adaptation of the parallel capacity ratiometer of Fig. 10-81. Other 

 compensator bridges are likewise adaptable to dual-coil ratiometer con- 

 struction. 



Comparison of in-phase components at successive stations may likewise 

 be made with two coils, but this process does not require a compensator. 



Fig. 10-107o. Dual-coil instrument for 

 measuring intensity ratios and phase dif- 

 ferences. Adaptation of capacity ratio- 

 meter. 



