130 O. U. VONWILLER. 



that in the other. When the cell was illuminated by an 

 8 c.p. lamp at a distance of 100 cms. similar relationships 

 were observed, the conductance being in all cases almost 

 exactly 1'3 times as great as when in the dark. 



An examination of the cell showed that one of the two 

 platinum wires had become detached from the selenium 

 over more than half its length; when the current entered 

 the selenium by this wire smaller conductance was obtained. 



Further experiments were carried out in which there 

 was a much greater difference between the areas of the 

 two surfaces of contact between metal and selenium, and 

 it was found that in most cases a distinct rectifying pro- 

 perty was to be detected. The best results were obtained 

 when the current passed between one of the platinum wires 

 of a cell, such as that mentioned before, and a needle or 

 thin platinum wire of which the end rested lightly on the 

 selenium. The rectifying power varied greatly, depending 

 largely on the pressure between the point and the selenium; 

 as the pressure was increased the conductance increased, 

 and the rectifying property decreased to a marked extent, 

 disappearing for comparatively small pressures. The 

 greatest ratio obtained for the currents for any e.m.f. was 

 about 40 to 1. 



Curve I in fig. 1 shows the variation of current with 

 e.m.f. for each direction of flow between one of the wires 

 of the cell described above, and a fine platinum wire the 

 end of which rested lightly on the selenium. The abscissae 

 represent the e.m.f's. applied (0*2 volt being taken as the 

 unit) while the currents are represented, on an arbitrary 

 scale, by the ordinates. In this, as in all other cases to be 

 mentioned, the positive values represent those obtained 

 when the current flowed from the metal surface of smaller 

 to that of larger area. 



