Chap. 8] 



MAGNETIC METHOD 



335 



ployed. The tripod and compass of this instrument are the same as in 

 the vertical magnetometer except that the auxiliary magnets are used 

 in a horizontal and not in a vertical position. 



For a deviation of the theory eq. (8-18) may be used, making allowance 

 for the fact that the zero position of the magnetic system is vertical and 

 that the components a and d of the gravity arm are interchanged because 

 d is now parallel to the magnetic axis. Then 



tan (f = 



MH cos a — mga sin i 

 ilfH sin a cos t + MZ sin i + mgd sin i 



In the operating positions, a = and t = 7r/2, so that 



MH — mga 



tan ip = 



and the reading 



which may be written 



s — So 



So = 



MZ + mgd 



^ 2f( MH - mga) 

 MZ — mgd 



2tM 





(8-28a) 



(8-286) 



(8-28c) 



(8-28d) 



MZ — mgd 



where mga/M is the horizontal intensity Ho for which the system is ad- 

 justed, and 



21M _ l^ 

 MZ — mgd ~ ^h 



(8-28e) 



is the reciprocal scale value of the horizontal magnetometer. 



If the instrument is not oriented correctly, an appreciable error may be 

 introduced. If in eq. (8-28a), t = 7r/2 and if eq. (8-28c) is substituted, 

 we have for the difference of correct 

 reading, and reading in the azimuth 



H 



s^- s = - (cosa - 1). (8-29) 



The curve shown in Fig. 8-24 has 

 been calculated for a horizontal in- 

 tensity of 21,7907 and a scale 

 value of 157 in Golden, Colorado. 

 An azimuth error of 40' produces 

 an error of one-tenth of a scale 

 division; hence, the effect of mis- 



FiG. 8-24. Effect of azimuth changes on 

 horizontal magnetometer. 



