Prof. Minchin's Experiments in Photoelectricity. 229 



Seleno-Aluminium Cells, 



In the year 1880 Mr. A. Graham Bell utilized the property 

 of conductivity possessed by a modification of selenium which 

 had been previously found to be a conductor, and also the 

 property of variable conductivity when light of variable 

 intensity falls upon it. These properties, as is well known, 

 were utilized in the reproduction of sound by means of a 

 telephone and a battery in the external circuit of which was 

 placed a sensitive selenium conductor. This selenium 

 arrangement is usually called a " selenium cell," but a 

 selenium conductor or a selenium resistance is a much more 

 appropriate term. 



When the photophone was announced, selenium resistances 

 were made in this country first, I believe, by Mr. Shelford 

 Bidwell, who showed some of them at the last meeting of this 

 Society, and by means of them produced two results of 

 striking beauty, which I shall presently endeavour to repro- 

 duce by different means. 



Last year 1 set about constructing selenium cells, pro- 

 perly so called — that is, cells in which electromotive force is 

 produced by the action of light. The method adopted was 

 to take two small clean plates of any metal, to spread a thin 

 layer of the already recognized sensitive selenium on the 

 surface of one of them, and, connecting each with a fine 

 platinum wire, to immerse them in presence of each other in 

 a small glass cell containing a liquid. Thus a large number 

 of metals and a large number of liquids had to be tried for 

 the best result. Plates of platinum, silver, tin, copper, zinc, 

 bismuth, mica, glass, and other substances were tried with 

 various liquids. With copper the E.M.F. produced by light 

 was almost, if not quite, zero. All the others gave consider- 

 able results ; but much the best result was obtained with 

 plates of aluminium ; and for some time the liquid used was 

 one of the alcohols — preferably methylic. In the course of 

 a few days, however, the aluminium plates in alcohol were 

 found to be covered with a kind of gelatinous deposit, which, 

 I am told, is an aluminate of alcohol. This liquid was, there- 

 fore, abandoned ; and the best result of all was found to be 

 produced with acetone. 



The process of forming the sensitive plate is as follows :— 

 On an iron tripod is supported a porcelain plate which is 

 heated from below by a Bunsen flame ; the little strip of 

 cleaned aluminium is placed on this plate, and when it has 

 got hot, one end of it is held in a forceps, while a drop of 

 melted selenium placed at the end of a very hot glass rod is 



Phil. Mag. S. 5. Vol. 31. No. 190. Mar. 1891. S 



