MAGNETIC METHODS 



117 



knife-edges and clamped against two centering pivot guides. The scale 

 circle is graduated in degrees of arc so that the position of the magnet can be 

 measured. The thermometer is mounted in the case to provide data for tem- 

 perature corrections. A light wire arm, operated from the back of the case, 

 moves the swinging assembly to its zero position. Orientation of the instru- 

 ment in the field of the magnetic meridian is accomplished by means of an 

 auxiliary compass which is placed on the tripod table. After proper orienta- 

 tion the compass is removed and the Superdip fastened to the tripod table. 



Figure 43 is a graph of the practical working sensitivity of a typical 

 Superdip. The curve marked 2 = 0° is of interest. Theoretically it should 

 coincide with the zero abscissa ; its departure therefrom indicates the amount 

 of the departure from infinite sensitivity introduced by the mechanical 

 features of the instrument, including friction. Although the sensitivity 

 curves for the various values of 2 are curved rather than straight, a 

 simple linear coefficient of sensitivity is sufficiently accurate — e.g., 12^/2 

 gammas per scale division for a 2 of 1°. 



120 «yxr 



O lO 2030«W BO<S01060 00 100II0I20 



CHANGE OF IN3TRUMErrr REAOINQS IN SCALE DIVISIONS 



Fig. 43. — Sensitivity of Hotchkiss Superdip. (Stearn, A.I.M.E. Geophysical Prospecting, 1932.) 



Preliminary Adpishnent of Superdip Magnetometer in the Field 



(1) The tripod table is leveled, and oriented by means of the compass. 



(2) The counterarm is set parallel to the magnet, and the angle of inclination 

 for the area is measured by using a small auxiliary scale. 



(3) The counterarm is set at the desired sensitivity (sigma value). 



(4) The counterweight is adjusted until the position of rest of the assembly is 

 approximately at right angles to the magnetic inclination. 



