LIGHT AND ELECTRIFICATION. 183 



charging with positive which so many experimenters have 

 observed, and which we may assume at present to be 

 correct ; though whether it is a true and simple effect we 

 will not at present consider quite decided. 



The inside of a metallic vessel is not affected by light, 

 even when the air contents are blown out and exchanged 

 for fresh air. Unless the atmosphere is thus changed 

 the experiment is inconclusive, because, even if a charge 

 were to leave the interior surface of the vessel and pass 

 to the air in contact with it, an electroscope connected 

 to the vessel would of course show nothing ; the charge 

 must be extracted entirely from the vessel before its 

 potential is affected. However, if proper precautions are 

 taken about blowing in non-electrified air (by no means an 

 easy matter), the experiment is perfectly negative ; no dis- 

 charging action occurs from a surface at zero density. 



If the beam of light which has been shining on the 

 interior of a charged pewter pot is transfered and made to 

 impinge on an outer edge of the same vessel the leak of 

 electricity immediately becomes rapid. 



On the other hand, a surface connected to the earth 

 (and therefore at zero potential) pours away its induced 

 charge rapidly when a beam of suitable light falls upon 

 it. And, indeed, this is a favourite and easy mode of 

 testing the power of different substances ; viz., by expos- 

 ing them to the inductive influence of a charged body, 

 such as a layer of wire gauze, through which light has 

 easy access, arid observing the rate of leak either by the 

 rise in potential of an electroscope attached to them 

 and initially at zero, or by a very sensitive galvanometer 

 inserted in the circuit which connects them to earth. In- 

 deed, with a water battery or other source of high EMF. 

 nothing further is needed than a sensitive galvanometer 

 and the two surfaces facing each other across a narrow air 

 space, which under the action of light becomes virtually 

 conducting. 



EFFECT OF MAGNETISM. 



One of the most singular observations on the subject 

 was made by Elster and Geitel, who placed the poles of a 



