Chap. 8] 



MAGNETIC METHOD 



359 



oc y 



consisting essentially of a circular coil of wire whose rotation in a magnetic 

 field produces an induction current except when the axis of rotation is 

 parallel to the lines of force. 



For use in earth magnetic work, the coil is so arranged that its axis m^-y 

 be placed into any direction in space. . 



The earth inductor (see Fig. 8-36) consists of (1) a horizontal circle, A, 

 mounted on a tripod base and rotatable about a vertical axis; (2) a frame, 

 B, connected to A by two short 

 axles, a and a'; (3) the coil C, sus- emf. 

 pended in B in the axis W . The 

 circle A is used to change the 

 azimuth of the coil axis ; the frame 

 B serves to change the inclination 

 of the coil ; and axis W is always 

 perpendicular to axis aa' . At- 

 tached to the frame is a graduated 

 vertical circle (concentric with aa') 

 on which the inclination of the 

 frame B may be measured (see 

 also Fig. 8-38). One end of the 

 coil shaft carries a commutator 

 consisting of two segments. The 

 corresponding brushes are at- 

 tached to the frame and are so 

 placed that commutation takes 

 place when the plane of the coil is 

 parallel to the axis of the frame B. 

 The brushes are connected to a 

 galvanometer. For measurements 

 of field direction, any type of gal- 

 vanometer of sufficient sensitivity 

 (string, loop, astatic, or the hke) 

 may be used. For intensity 

 measurements, ballistic galvan- 

 ometers or potentiometers are employed. 



For a derivation of equations applicable to direction and intensity 

 measurements, let the direction x" coincide with the axis of rotation of 

 the coil. Then y" is normal to x" in the plane of the frame; it is the axis 

 about which the frame is tilted; and it is always horizonjtal. The com- 

 ponent z" is always normal to the plane of the frame. The component 

 of the earth's field x" produces no induction, since it is in the axis of rota- 

 tion; only the components y" and z" directions are effective. If <p is the 



Fig. 8-37. Inductions due to z" and y" com- 

 ponents in earth inductor coil. 



