1881.] 071 Selenium and its applications to the Photophonej dc. 527 



tho surface of the wood, a Rcrow of from thirty to forty threads to the 

 inch is cut njion tho cylinder. On romovin<jj tho mica from tho wood 

 its two edges are found to be beautifully and regularly notched. Tho 

 first wire is then wound into alternate notches, and tho second into 

 the others. 



I will throw upon the screen tho image of a slip of mica with tho 

 two wires wound upon it, and ready ft)r the reception of tho selenium 

 coating. It will bo seen tliat tho turns are perfectly regular, and 

 close as tho wires are to each other, they do not touch at any point. 

 (Fig- 1.) 



Fio. 1. 



Mica Plate, wound with Two Copper Wires ready for Selenium Coating. 



Tho next step is to apply tho selenium, and to do this properly is 

 an operation which requires a certain amount of practice and j^atience. 

 The mica is heated to a temperature slightly above 217° C, and 

 melted selenium is spread over its surface as evenly as possible with 

 a metal spatula. The cell is then cooled, and its surface should be 

 smooth and lustrous. Before you is an embryo cell which has reached 

 this stage of its preparation. The selenium being still in the vitreous 

 condition, is a perfect insulator, and when the cell is connected in 

 circuit with a battery and a reflecting galvanometer, the spot of light 

 is found to be absolutely motionless. I now propose to crystallise 

 the selenium in your presence. The mere crystallisation occupies a 

 very short time. It is only necessary to place the cell upon a brass 

 plate, and raise it by means of a Bunsen burner to a temperature 

 somewhat below the fusing point of crystalline selenium. The method 

 described by Professor Adams in his classical paper published in the 

 * Philosophical Transactions,' is entirely different. He heated a bucket 

 of sand by placing in it a red-hot iron ball. At the expiration of an 

 hour he removed the ball, and placed in the heated sand his pieces of 

 vitreous selenium, wrapped up in paper. After remaining for twenty- 

 four hours, the selenium was generally found to have attained the 

 crystalline form, and the resistance of some of his specimens thus 

 prepared was far lower than that of any which have been made by 

 myself. Their sensitiveness, however, does not appear to have been 

 great. The method of crystallisation which I generally adopt, and 

 which is due to Professor Graham Bell, has at all events the merits 

 of simplicity and rapidity. In two or three minutes the whole surface 

 of the selenium film becomes dull and slate-coloured, and if, when the 



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