Chap. 8] 



MAGNETIC METHOD 



301 



lent to determine the induced magnetization and hence the susceptibil- 

 ity in the earth's field since the specimens are then tested in the lab- 

 oratory under the same conditions as prevail in nature. 



In the so-called test-tube method, the 

 sample is pulverized, placed into a 

 test tube of known volume, and sus- 

 pended near the needle of a unifilar 

 magnetometer as shown in Fig. 8-3a 

 or near one of the needles of an astatic 

 magnetometer^ (see Fig. 8-36). The 

 deflection measured by a telescope and 

 scale is proportional to the magnetic 

 moment, hence to the intensity of mag- 

 netization and the susceptibility of the 

 specimen. Thus H tan ^ = Ck, where 

 H = horizontal intensity; C = a con- 

 stant depending on the inducing field 

 component, volume, and distance of 

 the tube; and «p = the deflection angle. 

 The deflection may be increased by 

 making H small, that is, by so placing 

 a compensating magnet beneath the 

 magnetometer that its magnetic field 

 opposes and nearly equals the horizon- 

 tal component of the earth's magnetic 

 field (see Fig. 8-3a). 



In Koenigsherger's method, specimens 

 are used in solid form at a distance 

 which is small compared with the di- 

 mensions of the needle, and with the 

 magnetometer shown in Fig. 8-3a. 

 For field tests, a Schmidt vertical mag- 

 netometer is more suitable (after re- 

 moving the cork cover and placing the 

 sample on the instrument case). Fig. 

 8-4 shows the arrangement of a speci- 

 men with reference to the magnetic 



system of a vertical magnetometer. It should have the form of a disc 

 (thickness h, diameter d). Its distance a should be less than 1/5Z {I = 

 total length of the system)/" The strength of the magnetic pole induced 



Colorado School of Mines 



Fig. 8-3c. Unifilar (Koenigsberger) 

 magnetometer. 



^E. A. Johnson and W. F. Steiner, Rev. Sci. Instr., p. 237 (July, 1937). 

 ** This condition may be met for a laboratory instrument but not for the Schmidt 

 vertical magnetometer. 



