CONDUCTION OF ELECTRICITY IN MOLYBDENITE. 221 



is constant, its value not being altered by changes in the 

 maguitnde and direction of the current unless changes of 

 temperature are produced. It was also shown that the 

 rectifying effect, the change in resistance on reversal of 

 current flowing from metal to silicon to metal, is due 

 altogether to the existence of contact resistances at the 

 regions where the current enters and leaves the silicon, the 

 magnitude of such a resistance depending on the area of 

 the surface of contact, the pressure between the two 

 materials and the direction of flow. 



Experiments made since with other materials, including 

 galena and copper pyrites, which possess the rectifying 

 property, gave similar results, the actual resistance of the 

 material being small and invariable if the temperature does 

 not change, and the total resistance of preparations of these 

 crystals being made up mainly of two contact resistances 

 which vary with changes in magnitude and direction of 

 current in the manner described in the case of silicon. 



During the last few years several papers have been 

 published having reference to the conduction of electricity 

 in molybdenite, a substance preparations of which show 

 the rectifying property. W. W. Coblenz 1 and others have 

 described investigations on the change in resistance of this 

 material on exposure to light, while A. T. Waterman 2 has 

 noted several peculiar electrical propertiesof this substance. 

 One of Waterman's conclusions is that "at ordinary room 

 temperatures the resistance is a function of the applied 

 potential difference and appears to approach the value of 

 infinity as the potential approaches zero," and as these 

 writers concern themselves simply with the over-all resist- 

 ance, including the contact resistances, if such exist, it 

 seemed worth while to ascertain whether with this material 



1 Bureau of Standards, Bull. 16, (Science Papers No. 398) 1920, etc. 

 - Phil. Mag. Vol. xxxin, 1917. 



