Prof. Minchin's Experiments in Photoelectricity. 215 



the plate becomes completely insensitive to light. Thus the 

 photoelectric result is obviously due to some layer on the 

 surface. 



In connexion with the change of sign of the E.M.F. pro- 

 duced by continuous exposure, the following remarkable 

 experiment was made. Round the outside of a cylindrical 

 porous pot was fixed a coating of tin foil ; inside the pot was 

 placed a strip of tin foil, these two strips being the exposed 

 and unexposed plates of the cell respectively. The porous 

 pot was filled with water and immersed in a glass beaker also 

 containing water, the diameter of the beaker being very 

 slightly greater than that of the porous pot. The strip of 

 foil inside the pot was completely covered from light and 

 connected with one terminal of a galvanometer ; the foil out- 

 side the pot being connected with the other terminal. Out- 

 side the glass beaker, and fitting round it very closely, was a 

 cylinder of black paper with a vertical slit cut in it, the 

 breadth of the slit being about half an inch, while the diameter 

 of the cylinder was about 3 inches. Thus, by rotating the 

 black paper cylinder round the beaker, different strips of the 

 tin foil on the outer surface of the porous pot could be 

 successively exposed to light. 



The slit in the paper occupying a given position, the corre- 

 sponding portion of the tin foil was exposed to sunlight. The 

 current indicated that the plate was positive, and the exposure 

 was continued until the current changed its direction, i. e., the 

 plate became negative. The slit was then moved opposite 

 another and distant portion of the tin foil, which, by the same 

 process, was finally rendered negative ; and so on all the 

 way round. Thus the surface of the tin foil was divided into 

 a number of strips, which were alternately negative and 

 positive in their electromotive forces when exposed to light ; 

 and by rotating the black paper continuously round the 

 beaker, a series of currents in contrary directions were 

 obtained from the action of light on one and the same metallic 

 plate — a result which, at first sight, sounds very strange. 

 It is easily understood, however, when we remember the 

 different conditions into which the various strips of the 

 surface of the plate — i. e., some very thin stratum on the tin — • 

 were put in the preliminary process ; and, moreover, it pre- 

 pares us for cases in which different portions of a tin plate 

 which has been sensitized by a special process give, on 

 exposure, electromotive forces of different signs. 



I have not yet spoken of any process for producing a 

 sensitive layer on the surface, and a long time elapsed before 

 I discovered one. The tin foil hitherto spoken of is pure 



R2 



