MAGNETIC METHODS 113 



horizontal plane permits automatic alignment with the magnetic meridian. 

 A counterweight is provided to compensate the effect due to the vertical 

 intensity. 



Because this mining compass is affected by the horizontal as well as 

 the vertical intensity, its action is somewhat involved. In practice, the 

 instrument is used for qualitative observations, and H and Z are not 

 measured separately. Despite this limitation, it has been used extensively 

 in prospecting and for preliminary magnetic surveying. 



The modern dip needles commonly employed for reconnaissance sur- 

 veys are more accurate than the Swedish mining compass. |* 



Thalen-Tiberg Magnetometer. — This instrument may be used to 

 measure the horizontal intensity and the vertical intensity. In the former 

 case, it functions as a deflection magnetometer and in the latter case as 

 a magnetic balance. Essentially, the instrument consists of a magnetom- 

 eter which is rotatable about a horizontal axis t and may be placed either 

 in a horizontal or vertical plane. The magnetic needle system comprises a 

 magnetized needle which carries a sliding weight and is pivoted at its 

 center between two hardened steel bearings to permit free rotational move- 

 ment. The needle can be adjusted to balance horizontally by altering the 

 position of the sliding weight. A deflecting magnet of known magnetic 

 moment is supported at any desired distance from the pivot by a fixed 

 graduated horizontal arm. The instrument is supported on a light tripod, 

 and means are provided for accurate leveling. The sensitivity of the instru- 

 ment can be varied by altering the position of the center of gravity of the 

 magnet system with respect to the axis of suspension. 



The declination can be measured by orienting the instrument in the geographical 

 meridian. For field intensity measurements, the operation of the instrument is essen- 

 tially as follows. The deflecting magnet is attached to the horizontal arm ; this arm 

 is placed at right angles to the magnetic meridian and the magnetic needle is re- 

 leased. The needle deflects to an azimuth 6 where the turning moment due to the 

 magnet counterbalances that due to the earth's field. The deflection 6, the magnetic 

 moment Ma of the deflecting magnet, and the distance r to the pivot can all be 

 measured ; hence, the horizontal component H of the earth's field can be calculated 

 from the tangent formula, H = 2Md/r^ tan ^ (approx.). The horizontal intensity can 

 also be calculated from the sine formula, H =z 2Md/r^ sin d (approx.). The procedure 

 in this case is to rotate the instrument about its vertical axis until the deflecting mag- 

 net and magnetized needle are mutually perpendicular and to observe the deflection d of 

 the needle as it returns to the meridian when the deflecting magnet is removed. Both 

 the tangent and the sine methods yield the total horizontal field. The value of the mag- 

 netic anomaly is obtained by subtracting the normal field at the chosen base station 

 from the field measured. 



To measure anomalies in vertical intensity, the magnetometer is placed in a 

 vertical plane at right angles to the magnetic meridian and is balanced so that it is 

 horizontal in the normal earth's field. If the instrument is then set up at successive 

 stations in the field to be surveyed, the tangents of the angles made by the magnetized 



t N. H. Steam, "The Dip Needle as a Geological Instrument," A.I.M.E. Geophysical Pros- 

 pecting, 1929, pp. 345-363. 



* If the suspension point coincided with the center of gravity, the instrument would 

 be a dip circle, i.e., a device for measuring the inclination of the earth's field. 



t The description given here follows Broughton Edge and Laby, Geophysical Prospecting 

 (Cambr. Univ. Press. 1931). p. 182. 



