MAGNETIC METHODS 159 



adjustable stops is clamped at this position to provide a reference mark. 

 Next, the bar magnet is rotated in the opposite direction until the com- 

 pass needle is again deflected through 90°, and the second adjustable 

 stop is clamped at this position. The supplement 6 of the mean value of 

 the angle of rotation of the magnet is computed. The horizontal com- 

 ponent Hq of the earth's field at the base station and the horizontal 

 component F of the field produced by the bar magnet are related by the 

 equation 



Ho = F cos d (61) 



The instrument is now moved to a new (field) station and leveled as 

 before. If the horizontal intensity H at the field station is less than Hq 

 a rotation of the bar magnet to its stop will deflect the compass needle 

 through an angle of magnitude 90°+ 4>. The components of H and F 

 perpendicular to the plane of the resultant field are equal ; that is, 



H cos(t> = F cos (6 + 4>) (62) 



On replacing F by Ho/cos 6 (Equation 61), 



H = Ho(l -tane tan <^) (63) 



Thus, if Ho and are known for the base station, the relative variation 

 in the horizontal intensity can be calculated by observing ^. 



The angle (j> in an ore-free district will vary only by one or two degrees ; 

 the variations over a magnetite deposit, however, may be considerable. 



Fig. 67. — Gauss meter (A) with probe (B) and standard 

 magnet (C). (Courtesy of General Electric Company.) 



Gauss Meter. — An instrument has been designed for the purpose of 

 tracing out high intensity magnetic force or flux lines and mapping their 

 intensity.* This meter, illustrated in Figure 67, and shown diagramatically 



* See p. 74. 



