816 Mr. C. W. Heaps on the Effect oj 



Galenite, PbS. — Several bars o£ this substance were cut in 

 different directions from a large cubical crystal and mounted 

 as described above. Their resistances were small — about 

 0*2 ohm. It was found that the crystal behaved like an 

 isotropic substance and that the curves obtained resembled 

 those for the para- and diamagnetic metals. This was to be 

 expected, for in all its properties lead sulphide is very similar 

 to the metals. The addition of a sulphur atom to the molecule 

 of lead has the effect of increasing dR/R from 5 x 10 ~ 6 * to 

 4*5 x 10-4 for a field of 10,000. 



Pyrites, FeS 2 . — Specimens were prepared from this sub- 

 stance in the same way as from lead sulphide, but connexions 

 were made by copper plating and then soldering. The drift 

 of the galvanometer was unsteady when the bridge was in 

 balance, as if contacts were poorly made. The fluctuations of 

 the galvanometer made it necessary to reduce the sensibility of 

 the apparatus, but if the magnetic field had any effect on the 

 resistance, dRjR for a field of 10,000 must have been smaller 

 than 2 x 10 ~ 5 . The presence of contact resistance in lead 

 sulphide served to increase the effect of a magnetic field. If 

 it has the same result in pyrites probably the effect of a 

 magnetic field could be observed if dR/R were considerably 

 smaller than 2 X 10~ 5 . 



Pyrites can be magnetized, along one axis at least, so that 

 it is an example of a ferromagnetic substance the resistance 

 of which is unaffected by magnetization. 



Molybdenite, MoS 2 . — This mineral was secured through the 

 kindness of Professor G. W. Pierce of Harvard. As 

 molybdenite has a high specific resistance large slabs could 

 be used for experimenting. Contacts were made by copper- 

 plating and soldering, and here again an unsteady condition 

 of the galvanometer resulted when balance was secured, 

 showing that the specimen was undergoing irregular changes. 

 These changes were often as much as 10 per cent, of the 

 effect produced when the magnetic field acted. The curves 

 obtained indicated a decrease of resistance, and they seemed 

 to be affected very little by the direction of the field. It 

 seems possible that much of the resistance lay in the contacts, 

 so that the curve shown can hardly be taken as representing 

 the exact behaviour of the substance. In fact, Konigsberger 

 discarded molybdenite from his experiments because of the 

 difficulty of making electrical connexions — he found that a 

 continual increase of pressure at the contacts produced a 

 continual decrease of resistance. 



Magnetite, FeOFe 2 3 . — For rods of this mineral no trouble 

 * From Grunmacli's results. 



