Chap. 8] MAGNETIC METHOD 361 



The earth inductor may be used for determinations of declination or 

 inclination, or horizontal, vertical, or total intensity. 



To find the magnetic meridian^ the frame B and therefore the axis of 

 rotation of the coil are set vertical. The azimuth of the horizontal circle A 

 is varied until the current vanishes. (In eq. [8-43], z" is zero when 

 a = 0° and i = 90*'.) The accuracy of the meridian determination 

 depends on the setting of brushes and commutator. By comparing the 

 direction of zero current with the meridian direction as obtained from a 

 compass, the commutator angle may be determined. 



To find the inclination with an earth inductor, the axis aa' is oriented 

 in the magnetic prime vertical (a = 90°; z" = — H sin i + Z cos i). The 

 plane of the frame is rotated until the deflection of the galvanometer 

 vanishes (z" = 0; H sin i = Z cos i, or tan t = Z/H = tan I). The axis 

 of rotation is then in the direction of inclination. 



For determining the horizontal intensity, axis hh' is set vertical and axis 

 aa' in the magnetic prime vertical. Then a = 90" and t = 90** so that 

 z" = — H. For ballistic measurements, substitute this in eq. (8-45) so 



that Qh = —-^ (cos 5i — cos 52). By turning the coil through 180", 



5i = 0, 62 = 180", and 



where G is the galvanometer constant. 



To measure vertical intensity, the frame B is placed horizontally with 

 the coil axis toward north, that is, aa' in the magnetic prime vertical. 

 Then, a = 90" and t = 0"; hence, in eq. (8-43) z" = Z. Using a similar 

 procedure as outlined before, the vertical intensity as measured on a 

 ballistic galvanometer. 



In some earth inductors a spring switch is provided, which, after the 

 completion of one-half turn, cuts the coil from the galvanometer circuit. 

 Sources of errors in earth inductors are phase shifts due to coil inductance, 

 thermal e.m.f.'s due to brush friction, maladjustinent of brushes, and the 

 like. These are discussed in more detail by Dorsey. 



As shown by the above theory, the earth inductor is an almost universal 

 magnetic instrument and is adapted to the measurements of intensity 

 components and field directions. However, its predominant use at present 



" N. E. Dorsey, "The Theory of the Earth Inductor as an Inclinometer," Terr. 

 Mag., 18(1), 1-37 (March, 1913). 



