116 



EXPLORATION GEOPHYSICS 



with a counterweight) frictionally pivoted with the magnetized needle. 

 The theoretical sensitivity of the Superdip is determined almost entirely 

 by the angle between the counterarm and the needle. (Figure 41.) 



In practice, the limiting angle is established by the inclination of the 

 earth's field. For example, if the inclination i of the earth's field is 60° the 

 limiting angle A between the counterarm and the magnet would be 30° for 

 maximum sensitivity, because in this position the counterarm is horizontal 

 when the magnet is at right angles to the earth's field. Practically, how- 

 ever, maximum sensitivity is seldom desirable in field work, and the 

 angle L which is actually set off is smaller than the limiting angle. The 

 difference is called the sensitivity or 2 angle (S = 90°— i — L) and is 



adjustable to any desired degree of 

 sensitivity. 



A photographic view of the Hotch- 

 kiss Superdip is shown in Figure 42. 

 The dust-tight cylindrical case is made 

 of brass or aluminum and is mounted 

 vertically. It is about 6 inches in diam- 

 eter and 1^ inches thick and has a 

 removable glass face. The swinging 

 assembly which is mounted in the case 

 consists of the magnet, counterarm, 

 counterweight, and pivot, as well as a 

 release device, scale circle, thermome- 

 ter, and a mechanical adjustment finger. 

 The instrument is mounted on a sturdy 

 tripod which is equipped with quadra- 

 ture leveling screws, two levels (at 

 right angles), and vernier azimuth 

 adjustment. 



The magnetic system is a thin bar 

 magnet made of tungsten-cobalt steel. 

 The moving system swings in a vertical 

 plane and is mounted on a pivot. The 

 pivot is located at the center of gravity 

 of the magnet and rolls on a pair of 

 level jeweled knife-edges. The counter- 

 arm, which is attached to the same 

 pivot, may be adjusted to any desired 

 angular position with the axis of the 

 magnet. The counterweight is placed on 

 one end of the counterarm and is adjusted by varying its distance from the 

 pivot. 



The release device comprises two levers operated by a thumb screw, and 

 is so designed that the swinging assembly may be raised from the horizontal 



Fig. 42. — The Hotchkiss Superdip Mag- 

 netometer, a, non-magnetic case; b, magnet; 

 c, counterarm; d, counterweight; e, pivot; /, 

 release; g, scale circle; h, thermometer; i, 

 adjustment finger; ;. quadrature leveling 

 screvi's; k, levels; /, azimuth adjustment; m, 

 release screw. 



