686 



ELECTRICAL METHODS 



{Chap. 10 



In a single A.C. field, the voltage oscillates between its extreme positive 

 and negative values on a current line. The projection of its ampUtude 

 variation with time on a horizontal plane is a straight fine. The voltage 

 drop is a maximum along this line. At right angles thereto a true equi- 

 potential Une is present. Conditions are therefore the same as in the D.C. 

 fields. If another A.C. electrical field, or fields, interferes, one of the two 

 following phenomena will occur. If there is no phase shift between the 



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jm 



too 



10 \ 



8 



6 



2 \ 

 



Dtpfh in 

 Afefirs 



Scale 



Cesistivify in 

 Ohm -cm 



WO 



voo 



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mooo 



Fig. 10-34. Depth penetration (d at /o/e) for alternating current of various 

 frequencies, as a function of resistivity (permeability = 1 ; displacement current 

 neglected). 



fields, conditions remain as before. The maximum amplitude of the vector 

 is the resultant of the maximum amplitudes of its components. The pro- 

 jection of its time variation on a plane will still be a straight line. If, 

 however, the other field or fields are shifted in phase with respect to the 

 field considered, elliptical polarization of the resultant field occurs. This 

 occurrence derives its name from the fact that the projection of the time 

 variation of the resultant vector upon a plane is an ellipse. The out-of- 

 phase field may differ from the original field in direction or in coordinate. 



