MAGNETIC METHODS 



163 



The Factor of Grain Size. — Measurements of the magnetic suscep- 

 tibility of powdered samples of magnetite have shown that a decrease in the 

 particle size greatly reduces the effective susceptibility. The use of crushed 

 specimens is advantageous for study, and by preparing admixtures of mag- 

 netite dispersed in a non-magnetic material, a sample mixture is obtained 



PERCENT VOIDS 



Fig. 69. — General range of variation of susceptibility of magnetite and iron powders 

 with per cent voids in a field of 0.64 gauss. (L. B. Slichter, A.I.M.E. Geophysical Pros- 

 pecting, 1929, p. 247.) 



which approaches disseminations found in natural rocks. In a series of 

 experiments the per cent of voids was varied from 23 per cent to nearly 

 100 per cent with a resulting marked decrease in magnetic susceptibility, 

 as shown in Figure 69. In these tests, the iron and magnetite passed an 

 80-mesh sieve. 



It appears that the susceptibility of magnetite powder with 90 per cent 

 voids was about 0.26, and with 23 per cent voids 0.82, the latter being 

 about the practical limit obtainable in preparing samples. Projecting the 

 curve to zero voids, or the equivalent of solid magnetite, gave a value of 

 1.5, which is in general agreement with values of susceptibility for mag- 

 netite in solid form. 



The findings discussed above correlate with the tabulated values of 

 susceptibility. Those rocks in which the magnetite present is most widely 

 dispersed show the lowest susceptibility. 



