■Illumination and Electrical Conductivity in Selenium. AS3- 



effect of light on them is quantitatively reproducible again 

 and again. This renders it possible, as will be seen, to 

 measure the ultimate change of conductivity attributable to 

 illumination by the total increase of current resulting from 

 the exposure to light. This course seems to be the most 

 rational one. and it has the advantage that it enables obser- 

 vations to be made more quickly than when the recovery is 

 measured. For, as is well known, the maximum con- 

 ductivity of selenium when exposed to light is attained much 

 more rapidly than is the reduction to minimum conductivity 

 after the light is cut off. 



The justification for this method of procedure lies in the 

 fact that, for all the selenium cells examined, the conductivity 

 has been found to be a quite definite function of the intensity 

 of illumination. The cell has a certain conductivity in com- 

 plete darkness. When it is exposed to light, its conductivity 

 rapidly assumes a higher value, which becomes constant in two 

 or three minutes ; and when it is replaced in darkness, 

 although the conductivity decreases much more slowly, the 

 ultimate conductivity is not sensibly different from the 

 original " dark " value. One may therefore take as a 

 measure of the light effect either the. increase of conductivity 

 resulting from illumination, or the decrease caused by cutting 

 off the light ; and the former alternative is the more con- 

 venient in practice because it occupies less time. Care has 

 to be taken, however, to fulfil certain conditions. It has 

 been observed frequently that, in the entire absence of illu- 

 mination, the current in a selenium cell, to which a constant 

 considerable voltage is applied, increases progressively with 

 time. It is probable that this is mainly due to heating of 

 the selenium by the degeneration of electrical energy in it. 

 Certainly the conductivity increases rapidly with rise of 

 temperature. Whether this be the explanation or not is, for 

 the moment, irrelevant. The experimental fact is that, if the 

 voltage be constantly applied, and the current allowed to flow 

 continually during observations of the effects of illumination,, 

 the latter are masked or distorted to some extent by the 

 changes of conductivity attributable directly to the flow of 

 the current. It is possible to eliminate this latter effect 

 practically by permitting the current to flow in the selenium 

 only during the few seconds necessary to make an observation 

 of the conductivity. This is the procedure normally adopted 

 in the experiments here described, and it is in these circum- 

 stances that the simple law already mentioned appears to 

 hold with considerable accuracy. 



