1876.] 



the Action of Light on Selenium. 



115 



molecules wbich fcacilitates the subsequent passage of a current in the 

 opposite, but obstructs one in the same direction. Hence when two 

 currents are sent through successively, after a very small interval, in 

 the same direction, the resistance observed in the second case, even 

 with the higher battery-power, is often equal to or greater than it was 

 before. 



The results of these experiments seeming to indicate that the con- 

 ductivity of selenium is electrolytic, a number of experiments were 

 undertaken in order to discover whether after the passage of an electric 

 current through a piece of selenium any distinct evidence of polarization 

 could be detected. It was then found that, after passing the current 

 from a voltaic battery for some time through the selenium, and after 

 having disengaged the electrodes from the battery and connected them 

 with a galvanometer, a current, in some cases of considerable intensity, 

 in the opposite direction to that of the original battery-current, passed 

 through the galvanometer. This proved that the passage of the battery- 

 current sets up polarization in the selenium. 



All the results hitherto described v»^ere obtained with the selenium 

 kept in the dark. 



We then tried to discover whether on exposing the selenium to light 

 during the passage of the polarization-current any change in the inten- 

 sity of that current would be produced : we found that in several 

 cases there was a distinct change ; in most instances the action of the 

 light assisted the passage of the current ; but in one case we found that 

 the effect of light was not only to bring the deflection of the galvanometer- 

 needle down to zero, but also to send it up considerably on the other 

 side. 



Here there seemed to be a case of light actually producing an electro- 

 motive force within the selenium, which in this case was opposed to and 

 could overbalance the electromotive force due to polarization. 



The question at once presented itself as to whether it would be pos- 

 sible to start a current in the selenium merely hy the action of light. 

 Accordingly the same piece of selenium was connected directly with the 

 galvanometer. AVhile imexposed there was no action whatever. On 

 exposing the tube to the light of a candle, there was at once a strong de- 

 flection of the galvanometer-needle. On screening ofE the light the 

 deflection came back at once to zero. 



This experiment was repeated in various ways and with light from 

 different sources, the results clearly proving that by the action of light 

 alone we could start and maintain an electrical current in the selenium. 



All the pieces of selenium hitherto used had repeatedly had electrical 

 currents passing through them, and it therefore seemed desirable to 

 examine the eliect of exposure to light on pieces of selenium which had 

 never before had an electrical current sent through them. 



