INORGANIC RESPONSE i532 
say, is from the less to the more stimulated sensitive 
layer. On the cessation of light, there is fairly quick 
recovery (fig. 96, 0). 
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 ? 
