Chap. 8] 



MAGNETIC METHOD 



347 



horizontal. Equations previously derived for the mining compass ([8-38a] 

 and [8-38c]) fully express the action of the dip needle in the magnetic 

 meridian. 



Jewel Bearing 



Jewel Sear/ng 



(a) {b) 



Fig. 8-29. (a) Dip needle in gimbal suspension. (6) Ordinary dip needle. 



Fig. 8-30. Thomson-Thalen magnetometer (after Haanel). 



The Thomson-Thalen magnetometer is similar in construction to the 

 Schmidt vertical balance. Anomalies in vertical intensity are not read 

 in terms of deflection angles; they are compensated by changing the posi- 

 tion of a deflecting magnet beneath the magnetic system. The differences 



