304 



MAGNETIC METHOD 



[Chap. 8 



^ 



4mperemeter 



Seconifary 



\Batttry 



F = - (H' — Hi), (H = field intensity at lower end of sample; Hi = field 

 intensity at upper end of sample) is balanced by Q grams, the susceptibility 



. = ^g:^„ ■ (8-10) 



H2 TT2' 



— Xii 



in which Hi can be neglected in comparison with H when the specimen is 

 of sufficient length. 



The inductive methods for susceptibility determinations are based on the 

 fact that the inductance of a coil and the energy transfer in a transformer 

 depend on the magnetrc permeability of the core material. In the 

 apparatus designed by Stutzer (Fig. 8-6) the primaries of two air-core 

 transform.ers are connected in series-aiding to an ammeter, chopper, and 



battery, while the secondaries 

 are connected in series-oppos- 

 ing through a galvanometer 

 and commutator mounted on 

 the same shaft with the 

 chopper. The coils are ac- 

 curately balanced so that 

 when pulsating current is sent 

 through both primaries, no 

 current flows in the second- 

 ary circuit if no specimen is 

 in the test coil. The balance 

 is disturbed when a sample is 

 inserted in the test coil. The 

 resulting galvanometer de- 

 flection is proportional to the 

 susceptibility of the sample. 

 This method is very accurate for minerals of small susceptibility. Strongly 

 magnetic substances should be mixed with quartz sand, and the like, and 

 a corresponding correction should be applied. 



Wm. M. Barret,^ in his inductive method, has employed a single impulse 

 through the primary (or primaries) of one (or two) inductance coils whose 

 secondary (or secondaries) is (or are) connected to a ballistic gal- 

 vanometer. The circuit is shown schematically in Fig. 8-7. The current 

 from a battery, B, passes through a reversing switch, <Si , to a key, Ki , 

 by which the primary circuit is opened or closed. It then passes through 

 the switches, Sz and S3 , to the fixed mutual inductance, M/, the vernier 

 inducta.nce, Mv , and the ammeter, ^42 , in one circuit and through the 

 mutual inductance, Mt (containing the sample), and the ammeter, A3 , in 



Primary 



Ga/tvrfo(7') 



/neffr rr 



-mm 



pnm. 



Condenser 

 lOfif 



Fig. 8-6. Stutzer's inductive arrangement for 

 susceptibility determination. 



A.I.M.E. Geophysical Prospecting, 216 (1932). 



