Chap. 8] MAGNETIC METHOD 351 



angle is magnified by compensating most of the horizontal intensity. To ob- 

 tain H with an accuracy of a few gammas by customary methods, it is neces- 

 sary not only to use a comparison field of comparable magnitude at right 

 angles to H, but to observe deflection angles with a high degree of accuracy. 

 Obviously, the same accuracy may be obtained by compensating a portion 

 of H, comparing the remainder with only a small field at right angles to H, 

 and measuring deflection angles by crude means. The normal horizontal 

 intensity is compensated in this instrument by two magnets north and 

 south of the compass box with their north poles oriented as shown in Fig. 

 8-32. Above the compass box, in third Gauss position, is a small magnet 

 at right angles to the direction of the compensating field. Then AH = 



F 



Hi — Ho and tan ip = , so that 



Hi - Ho 



where M\ and M^ are the moments of the compensating magnets, ilf 3 that 

 of the small comparison magnet, n , r2 , and rs their distances, and ki , k2 , 

 and ks their deflection constants. 



De Collongue deflectors have been constructed for horizontal and vertical 

 intensity determination. The horizontal instrument is somewhat similar 

 to the Schmidt compensation magnetometer. It consists of a compass 

 box, a small deflector in the third Gauss and E-W position above the 

 needle, and a large compensator, likewise in the third Gauss position, 

 above the deflector. The compensator is always in the magnetic meridian 

 and is moved up or down until the magnetic needle points E-W. Thus, 

 the inverse cube of its distance is proportional to the horizontal intensity. 

 The vertical intensity deflector is similar to the Thomson-Thal^n magne- 

 tometer and has been developed primarily for observations on board ship.'* 

 A compass card is equipped as usual with magnets but is not balanced by a 

 counterweight, and the vertical intensity is compensated by a deflector 

 underneath. As the ship travels from one point to another with a different 

 vertical intensity, the compass card will show an inclination which may 

 be compensated by moving the deflector. Thus, vertical intensity ob- 

 servations are made here in the magnetic meridian. 



The Ostermeier universal magnetometer is, in principle, identical with the 

 Kohlrausch magnetometer. The magnetic system of a Schmidt vertical 

 magnetometer is used as the deflector, and the compass of the same 



^* Illustrated in A. Nippoldt, Erdmagnetismus, Einfuehrung in die Geophysik, 

 II, 37 (Berlin, 1929). 



^* Illustrated in Nippoldt, op. cit., p. 41. 



