126 



EXPLORATION GEOPHYSICS 



and of these the 0, the 20, and the 40 division Hnes were longer and heavier 

 than the others. This is illustrated in Figure 52. 



1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 



1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 



Fig. 52. — Scale in old style magnetometers as it 

 appears with system arrested. 



In reading, the 20 mark, or center, of the reflected (moving) scale is 

 used as an index on the stationary scale. This is shown in Figure 53(b). 

 The tilt of the magnetic system at a station may be so great that the 

 20, or center index, is not on the stationary scale, in which case the or 

 the 40 index may be used, as in Figure 53(c). With these different 

 indexes, the range of the scale is from — 20 to + 60 or a total of 80 

 scale divisions. 



(d) 



(b) 



II 1 1 I I II I II I I III 



iiii | iiii iiii|iiii 



iiii [i iii ii i i | iii i 

 iiii | i i i i iiii|i i ii 



1 1 1 1 1 I I 1 1 I I 1 1 1 1 1 1 I I II 1 1 II I 



n il ill! n il III! III! n il n il n il 



READING 20.0 



READING 7.« 



(c) 



i|i in liii|i 



1 1 1 1 1 n n 1 1 1 1 1 n 1 1 



READING 49.4 



Fig. 53. — Examples of 3 scale readings of older type magnetometers. Upper scale is 

 the moving scale. (Scales offset, to illustrate method of reading.) 



The fixed and moving scales of Figure 53 show only the method of 

 reading. In looking into the telescope of the magnetometer the two scales 

 coincide closely. The moving scale appears to be superimposed on the 

 fixed scale and seems to float along it as the system tilts. 



When the magnetic system is at rest horizontally (zero tilt angle), 

 the center of the scale, or 20 mark of the reflected scale, coincides with 

 the center or 20 mark of the stationary scale. (See Figure 53a.) When 

 reading the instrument, if the reflected (or moving) scale runs toward 

 the north as marked on the outer case by an N, the tilt angle of the north 

 end of the magnetic system is below a horizontal surface through the 

 bearings on which the system rests. In this case (in the northern hemi- 

 sphere) the reading is considered as plus or greater than 20. If the 



