Magnetic Fields on Resistance. 819 



complex in structure than pyrrhotine, which does not belong- 

 to the regular system. Weiss and Quittner used relatively 

 small fields, but their results seem to indicate magnetic 

 isotropy after saturation is reached. It is of interest to note 

 that this anisotropic character of magnetite as shown by the 

 change of resistance in a magnetic field persists long after the 

 saturation point is passed. 



In general, all the results obtained may be summarized as 

 follows : — 



1. For para- and diamagnetic metals a transverse field 

 produces a greater increase of resistance than a longitudinal 

 field, and for small magnetizing forces the increase is pro- 

 portional to the square of the field strength. 



2. Crystals of lead sulphide in a magnetic field behave 

 like an isotropic para- or diamagnetic metal. 



3. The resistance of iron pyrites is unaltered by a magnetic 

 field — at least, if there is a change, it is smaller than 2 x 10" 5 

 per ohm for a field of 10,000. 



4. Whatever the direction of the magnetizing force, the 

 resistance of molybdenite decreases, and it decreases in much 

 the same way as does the resistance of Heusler alloy in a 

 longitudinal field. 



5. The behaviour of magnetite in a magnetic field indicates 

 that this mineral has a very complex internal structure ; it 

 has different properties along different axes even when sub- 

 jected to magnetizing forces much greater than are necessary 

 to produce saturation. For fields greater than four or five 

 thousand, after the magnetization has reached a maximum, 

 dR/H and H are always connected in a linear relation. 

 Indeed, this seems to be the case for everything examined, 

 provided a large enough field can be secured. 



As far as the crystalline structure of metals is concerned, 

 one could say that for a non-magnetic substance crystallizing 

 in regular form, the internal crystalline structure would have 

 no effect on the change of resistance in a magnetic field. For 

 ferromagnetic metals, however, this crystalline character 

 might play a rather important role, since the orientation of 

 the minute crystals might influence the general change of 

 resistance caused by magnetization. 



It is again a pleasure to thank Professor E. P. Adams for 

 his advice and for the helpful interest he has taken in the 

 work. 



Palmer Physical Laboratory, 



Princeton University, 



Princeton, N.J. 



