CONDUCTION OF ELECTRICITY IN MOLYBDENITE. 227 



these cases it is frequently found that irregular changes 

 occur, and it is thought that the high values are not due 

 to actually higher values of resistivity but to discontinuities, 

 sometimes temporary in nature, in the thin sheets or lamellae 

 of which the substance consists. The over all resistance 

 A D may be increased to thousands or hundreds of thousands 

 of ohms by alterations in the contact pressure and area. 



In the paper quoted Waterman finds a relation between 

 the resistance and temperature. In view of the above 

 result it is likely that such a relation must refer mainly to 

 contact resistances and does not give any information con- 

 cerning changes in the resistivity of the material. Attempts 

 were made therefore to investigate the behaviour of such 

 preparations at various temperatures. Strips of molybdenite 

 furnished as before with copper contacts were placed in an 

 oven heated electrically, and the potential differences for 

 several currents measured at a number of temperatures. 

 Many specimens were tested over various temperature 

 ranges, the highest temperature used being about 200° O. 

 In all cases the results resembled those obtained at room 

 temperatures. Always Ohm's law held for the central 

 portion B O and the potential difference for an end pair of 

 contacts was greater when the current entered at that 

 end than when it left. The change of curvature noted 

 above near the beginning of the + A B and -OD curves 

 was observed in some cases and probably occurred in all. 



In every trial the potential difference between each pair 

 of contacts decreased with rise of temperature, but the 

 relative decreases were not the same for all. In figure 3 

 ■are given curves for various temperatures for + A D and 

 — AD and in figure 4 the corresponding curves for B C for 

 the same strip from which the curves of figure 2 were 

 obtained. The numbers placed near the lines give the 

 mean temperature of the preparation during the observa- 



