INORGANIC RESPONSE 153 



say, is from the less to the more stimulated sensitive 

 layer. On the cessation of light, there is fairly quick 

 recovery (fig. 96, b). 



The character and the intensity of E.M. variation 

 of the sensitive cell depend to some extent on the pro- 

 cess of preparation. The particular cell with which 

 most of the following experiments were carried out 

 usually gave rise to a positive variation of about 

 008 volt when acted on for one minute by the light of 

 an incandescent gas-burner which was placed at a dis- 

 tance of 50 cm. 



Typical experiment on the electrical effect induced 

 by light. This subject of the production of an electrical 

 current by the stimulus of light would appear at first 

 sight very complex. But we shall be able to advance 

 naturally to a clear understanding of its most complicated 

 phenomena if we go through a preliminary consideration 

 of an ideally simple case. We have seen, in our experi- 

 ments on the mechanical stimulation of, for example, 

 tin, that a difference of electric potential was induced 

 between the more stimulated and less stimulated parts 

 of the same rod, and that an action current could thus 

 be obtained, on making suitable electrolytic con- 

 nections. Whether the more excited was zincoid or 

 cuproid depended on the substance and its molecular 

 condition. 



Let us now imagine the metal rod flattened into a 

 plate, and one face stimulated by light, while the other 

 is protected. Would there be a difference of potential 

 induced between the two faces of this same sheet of 

 metal ? 



