MAGNETIC METHODS 



123 



passes through the objective lens and is reflected back by the plane mirror 

 fastened to the magnet system. Thus, two scales are seen superimposed 

 in the eyepiece of the telescope ; one is the direct and the other the reflected 

 image. The difference in position of corresponding marks on these scales 

 is a measure of the amount of the deflection of the magnet system. 



LIGHT 

 FROM SKY 



, EYEPIECE 

 li^^a-f^LENS 



1/2 PRISM 



TELESCOPE TUBE 



LATITUDE WEIGHT 



— Es-cn — 



-SENSITIVITY WEIGHT 

 Fig. 50. — Optical system, vertical component magnetometer. 



A vertical brass weight, supported on a threaded rod on the under side 

 of the magnet system, affords a means of varying the sensitivity of the 

 instrument. For ordinary subsurface structural investigations, the magnetic 

 system of the vertical balance is usually adjusted for a sensitivity of about 

 10 to 30 gammas per scale division. The scale in one type of instrument is 

 divided into 120 divisions ; hence, the intensity range of the instrument is 

 approximately 1200 to 3600 gammas. In areas of large magnetic disturb- 

 ances, the sensitivity is decreased, with a resultant increase in total range 

 of the instrument. 



As shown in Figure 51, the magnetometer is clamped on a special 

 adjustable wooden tripod, the graduated rotational head of which is leveled 

 by means of a center bubble and three leveling screws. Fastened to the 

 under side of the tripod head is an extension tube holder for an auxiliary 

 magnet. A thumb screw clamps the auxiliary magnet at any desired dis- 



