114 



EXPLORATION GEOPHYSICS 



needle with the horizontal will be proportional to the vertical intensity anomalies at 

 these stations. 



The Thalen-Tiberg magnetometer is most useful in regions where the vertical 

 intensity anomalies exceed 20 gammas. 



In another type of magnetometer based on the compass principle, the opposing 

 torque is supplied by the torsion of a helical spring instead of the field due to an 

 auxiliary magnet. One of the earliest instruments of this type, the Dahlblom pocket 

 magnetometer, comprises a compass mounted in a circular case, the latter being free 

 to rotate about a horizontal axis. Because the magnetic needle system can swing in 

 both a horizontal and a vertical plane, the magnetometer may be used to measure 

 either the horizontal or the vertical intensity, t A magnetometer which is similar to 

 the Dahlblom instrument, except that it measures the vertical intensity only, has also 

 been proposed, t 



The Hotchkiss Superdip. — The Hotchkiss Superdip magnetometer 

 combines in one instrument the functions of a dip needle and a mag- 

 netometer. Essentially, the superdip is an improved dip needle with adjust- 

 able sensitivity limited only by friction and other mechanical difficulties. 

 The operating principles may best be explained by comparing the ordinary 

 dip needle with the Hotchkiss Superdip.* 



Fig. 40. — Sketches illustrating working principles of the ordinary dip needle (a) and the Superdip 

 (b). (After Steam, A.I.M.E. Geophysical Prospecting, 1932.) 



The action of the ordinary dip needle in measuring the intensity of 

 the earth's field derives from the effect of the counterweight with which the 

 needle is provided. (Figure 40.) When the instrument is oriented in the 

 plane of the magnetic meridian, the counterweight serves to balance 

 the magnetized needle at some fixed angle to the direction of the earth's 



t E. Haanel, On the Location and Examination of Magnetic Ore Deposits by Magnetometric 

 Measurements (Ottawa, 1904), pp. 99-106. 



% C. A. Ileiland, Physics. Vol. 3, pp. 18-22, July, 1932. 



* N. H. Steam, "Practical Geomagnetic Exploration with the Hotchkiss Superdip," A.I.M.E. 

 Geophysical Prospecting (1932), pp. 169-197. 



