Chap. 8] 



MAGNETIC METHOD 



329 



the magnetic system may be adjusted approximately to the magnetic 

 latitude in his area (by shifting blades). After this adjustment has been 

 made, the steel blades should not be changed by the user, since the temper- 

 ature correction also depends on their position. On arrival, the magnetom- 

 eter may require readjusting of the latitude screws so that the reflected 

 scale is near 20 (or 30 for the 

 new telescopes) at a base sta- 

 tion in the area under survey. 

 For two widely separated 

 areas it is advisable to have 

 two properly adjusted sys- 

 tems. Where large anoma- 

 lies occur, the scale value 

 should be increased or auxil- 

 iary magnets should be used. 

 The scale value of a mag- 

 netometer may be determined 

 with (1) magnets, (2) coils, 

 (3) observations in different 

 azimuths. A new instrument 

 should first be tested for uni- 

 formity of scale value by 

 measuring and plotting the 

 variation of reading with field 

 changes produced by magnets 

 or coils. Routine scale value 

 tests with magnets are made 

 with a long deflection rod 

 which attaches to the tripod 

 head and carries a slide with 

 pivoted magnet holder. Scale 

 value determinations should 

 be made in the middle of the 

 scale. Readings near 20 may 

 be produced by a magnet set 



up on another tripod (in second Gauss position) or by using a deflector 

 attachment especially made for this purpose (see Fig. 8-20). The average 

 scale value should be adjusted to about 30 gammas for the earlier and to 

 15 gammas for the new systems. It is useless to make the instrument 

 too sensitive, as the errors are increased proportionally. In determining 

 the scale values by magnet deflection, a magnet of known moment Ma 

 is placed at a distance r on the deflecting rod. If the reading without 



American Askania Corp. 



Fig. 8-20. Deflector-attachments for cen- 

 tering reading and for scale-value determina- 

 tion (Askania). 



