Prof. Minchin's Experiments in Photoelectricity. 233 



tube and, coming out at its ends, this wire terminates in two 

 little spheres of platinum. Over the quadrants were fixed two 

 small mercury-cups which were completely insulated from the 

 quadrants and which were permanently connected with 

 the poles of an external battery which had an electric bell in 

 its circuit. When light shone on the photoelectric battery 

 (whose poles were connected with those of the electrometer), 

 the needle was deflected and, carrying the little glass tube 

 with it, brought the platinum points into contact with the 

 mercury in the cups ; thus the circuit of the voltaic battery 

 was completed, and the bell rung. This arrangement was 

 unsatisfactory, owing to the difficulty with which the plati- 

 num points separated from the mercury, and it has been 

 replaced by a much more satisfactory plan devised by Mr. 

 Appleyard, to whom I beg to express my thanks for the 

 untiring perseverance which he devoted to the perfecting of 

 the apparatus. 



The improved arrangement is as follows : — 

 In the figure N N represents the needle of the quadrant- 

 electrometer, the quadrants of which are not represented ; 



g q m is a very fine glass tube passing down through the 

 middle point, wi, of the needle; a platinum wire, pgqmr, 

 traverses this tube, the portion g q lying at right angles to 



