348 MAGNETIC METHOD [Chap. 8 



in the inverse cubes of the observed readings are, therefore, proportional 

 to the differences in intensities. The instrument shown in (Fig. 8-30^^), 

 contains a magnetic system consisting of two parallel cylindrical magnetic 

 needles, mounted in a brass ring with like poles in the same direction. 

 This ring is supported in agate, cup-shaped bearings by two pointed 

 hardened steel screws. The arresting mechanism consists of another 

 brass ring below the magnetic system. This ring carries four forks 

 which lift the system off its bearings and press it firmly against two pillars 

 mounted to the upper part of the circular case. The compensating mag- 

 net is slightly below the system, north pole up, and is attached to a 

 long spindle, which ends at the top of the case. The spindle may be 

 moved up or down by turning a keyed thumb-nut at the top of the case. 

 This nut is graduated in centimeters, part of the threads being faced off 

 for the divisions. The readings indicate the distance of the upper pole 

 of the magnet from the magnetic system. 



The instrument is used at right angles to the magnetic meridian, placed 

 in this direction by a compass mounted permanently on a long staff 

 above the instrument. The deflection of the magnetic system is given 

 by eq. (8-20), or tan ij = (MZ — mga)/mgd. Contrary to the Schmidt 

 balance, tj in this equation is very large, since mga is too small to com- 

 pensate MZ. Compensation is effected by a magnet producing, in its 

 normal position, a field AZo = — 2Afako/ro (which is only approximate for 

 this instrument, as the magnet is very close to the magnetic system). 



With the effect of the compensator, tan ?? = — -^ [^ ~ ~W ~ ^° ) ' ^^^ 



point with normal vertical intensity, the compensating magnet is so ad- 

 justed that for its distance n the system is horizontal. At another point 

 with the intensity Z, the compensator is moved to a distance r, so that 

 the system is again horizontal. Then Z' — Zo = 2Ma(ki/r? — ko/ro). 

 For small differences in distance, ko = ki = k, and 



AZ = 2Mak -3 - -3 . (8-39) 



This formula is only approximate. A graph showing vertical intensity 

 anomaly as a function of compensator distance based on a more accurate 

 calculation, has been published by Haanel.'^ In practice it would be 

 preferable not to use any calculated effect, but to calibrate with Helm- 

 holtz coils or deflecting magnets. 



In the Dahlblom pocket magnetometer a magnet is suspended in bearings 



" See Haanel (op. cit.), Figs. 26 and 27 on pp. 57-58, and plate E; also No. 507 s.e. 

 and No. 509, pp. 30 and 31, Berg's catalogue. 

 " Op. cit., p. 62. 



