MAGNETIC METHODS 



127 



moving scale runs toward the south end of the instrument, the tilt angle 

 is above the horizontal and the reading is less than 20. The use of a reading 

 of 20 for the horizontal position of the system or zero tilt angle is arbitrary. 

 In each case the amount of a reading is so many scale divisions from 20, 

 as shown in Figure 53. 



In the optical system of the later models of magnetometers the scale 

 has 60 divisions. Mounted to the side of this scale is a glass plate with 3 

 index lines. The double scale image is avoided, and by using the position 

 of the index lines on the scale to measure the tilt angle, a range of 120 

 scale divisions is obtained. 



In field work with the old style systems, the magnetic system is adjusted 

 by means of the latitude adjustment screws at the ends of the aluminum 

 block. In the new type systems, this adjustment is made by moving the 

 weight on the latitude spindle. In both cases, the adjustment is made so 

 that the instrument reads close to the 20 mark at the base station used in an 

 area, when set up and the system released. This adjustment must be 

 changed if a survey extends over an area so large that the normal increase 

 or decrease of magnetic intensity causes the reflected scale to go out of 

 view. 



Theory of Operation of Vertical Magnetometer. — The magnetic 

 system of the Askania vertical magnetometer is provided with three adjust- 



MAGNETIC EAST 



AXIS OF ROTATION 



CENTER OF GRAVITY 



Fig. 54. — Diagram showing relationships of different parts of 

 compensated magnetic system for the vertical magnetometer. Note 

 that the center of gravity has been shifted away from the axis of 

 rotation. 



able counterweights or masses Wi, W2, and W3 as shown in Figure 54, 

 illustrating the new-style, temperature-compensated system. The distri- 

 bution of the mass of the magnetic blades and of the aluminum block in 

 the older-style system, however, produced a like result, except that temper- 

 ature compensation was not complete. The same theory applies to both 

 systems. 



The combined effect of these weights, or the mass distribution between 



