232 Prof. Minchin's Experiments in Photoelectricity. 



cell was about one metre from the prism. Most probably, of 

 course, the number 101, corresponding to the border of green 

 and blue, is less than it should be. 



All other photoelectric cells may almost be said to be 

 sensitive to the blue alone. 



Effect of Continuous Exposure to Light. — The effect of 

 exposing a seleno-aluminium cell continuously to daylight 

 depends on whether the cell is left on open or on closed 

 circuit. A cell left on open circuit for many hours, and then 

 kept in the dark until its E.M.F. settles down, will be found 

 to have fallen to one fifth of its original value ; but if 

 then kept during the night and observed in the morning, 

 the E.M.F. produced by exposure to light will have quite 

 recovered its first magnitude. Not so with the cell on closed 

 circuit ; its E.M.F., on fresh exposure, will be reduced to 

 nearly one half its original value. 



A cell which had been exposed to daylight for five days 

 (with, of course, the advantage of each night's resuscitation) 

 is now shown to the Society, and its action can be compared 

 with that of a fresh cell. 



Connexion with a Daniel! during Exposure. — When a 

 Daniell with a very great resistance interposed, as described 

 in connexion with the tin cells, is connected with a seleno- 

 aluminium cell, so that the Zn pole is first connected with 

 the sensitive plate, and then with the insensitive, the E.M.F. 

 developed by light is much greater in the first case than in 

 the second — a result which is the same as before, since the 

 seleno-aluminium plate is the negative one in the cell, and 

 therefore the effect of this connexion with the Daniell is to 

 develop between the poles of the photo cell a difference of 

 potential opposed in sign to that which light produces. 



Mechanical Effects. — The E.M.F. generated by light in a 

 photoelectric battery can be utilized for ringing electric bells, 

 lighting or extinguishing electric lamps, and possibly other 

 things, although the materials of the battery are never used 

 up in producing currents. The method which I have em- 

 ployed consists in utilizing the motions of an electrometer 

 needle (due to the E.M.F. of the photoelectric battery) for 

 making a contact and completing the circuit of an ordinary 

 voltaic battery, whose current, thus completed, rings a bell, 

 lights a lamp, &c. 



The first arrangement which I employed for this purpose 

 was as follows : — At the middle of the aluminium needle of a 

 quadrant-electrometer, and at right angles to the length of 

 the needle, is fixed a very fine glass tube about 1£ inch 

 long ; a platinum wire traverses the interior of this glass 



