CONDUCTION OF ELECTRICITY IN MOLYBDENITE. 225 



This effect was first observed with silicon (loc. cit.) and 

 was found also with galena. The potential difference for 

 the pair of contacts where the current enters the material 

 increases at first at a greater rate proportionately than 

 the current. In a case like that of figure 2 where the 

 resistances for A B and O D differ greatly, we have a like 

 effect for the curve + A D or -AD, the former in this case 

 as the ordinates for A B are greater than those for O D, so 

 that the over-all resistance has a maximum value not for 

 zero current but for a small + current. Probably this 

 effect exists in all cases in which rectification occurs, but 

 frequently the currents and potential differences in this 

 region of the curve are too small to allow satisfactory 

 measurements to be made. 



(5) The curve BO is a straight line,. the same for currents 

 in each direction; this is not shown well in fig. 2 on account 

 of the scale selected, but readings taken with currents 

 twenty-five times as large as the largest given there show 

 no appreciable deviation from the linear relation between 

 potential difference and current. 



(6) The potential difference across BO is a very small 

 fraction of the total potential difference A D although the 

 four contacts are at roughly equal intervals, the distance 

 between A and B being 13 mm., between B and O, 11 mm. 

 and between O and D, 8 mm. 



The curves show that the total resistance consists mainly 

 of the contact resistances at A and D, a fact that is of 

 course well known for most materials of this class because 

 the resistance can be altered readily by changes in the 

 pressure of the contacts; we see too from (5) that the 

 actual conductivity of the material is constant for a wide 

 range of current values. When large currents are used 

 the curves AD, AB and CD become more nearly horizontal 

 the apparent resistance continually decreasing, and the 



O -December 7, 1921. 



