220 Prof. Minchin's Experiments in Photoelectricity. 



.Many liquids can be used in the cell ; but I have found 

 that the best results are obtained from methyl alcohol prepared 

 from oil of wintergreen. With methyl alcohol prepared from 

 wood-spirit I failed to obtain anything like the maximum 

 E.M.F. on exposure to light. A plate so prepared is, on 

 exposure to light, positive to the clean plate in the cell. In 

 some liquids the plate is almost quite insensitive to light, but 

 on replacing it in methyl alcohol its sensitiveness reappears. 



It is remarkable that the plate while immersed in the cell 

 (and unexposed) takes a considerable time — about five hours 

 — to develop its maximum sensitiveness ; that is, the cell 

 should be left in the dark for this time to allow the plate to 

 develop. Moreover, shortly after the plate has been formed, 

 it is much more rapid in its response to light and shade than 

 it is subsequently ; not that there is any falling off in the final 

 indications of E.M.F., but that the indications are more slowly 

 produced. 



Fig. 3 represents a cell connected with the electrometer. 

 AB is a small glass tube nearly full of the liquid ; P and Q 

 are the sensitized and unsensitized plates, two fine platinum 



Fiar. 3 



wires being either soldered to them or passed through minute 

 holes in them, these wires being sealed into the glass tube at 

 A and B, and connected with the poles m, n, of a quadrant 

 electrometer, E ; the arrow L represents the incident light. 



After forming the sensitive plate, it should be tried in the 

 cell before the cell is sealed up completely ; if it is not satisfac- 

 torily sensitive, it should be taken out and re-heated either on 

 the glass plate in fig. 2 or over the chimney of a paraffin 

 lamp. This further heating will often convert a partially 

 sensitive plate into a very sensitive one. 



The cell is usually fixed in a piece of cork by means of 

 which it may be held in a support. 



