154 BULLETIN OF THE 



these impurities are eliminated the selenium appears to be more 

 constant in its action and more sensitive to light. 



Prof. W. G. Adams* has shown that tellurium, like selenium, has 

 its electrical resistance affected by light, and we have attempted to 

 utilize this substance in place of selenium. The arrangement of 

 cell (shown in Fig. 7) was constructed for this purpose in the early- 

 part of 1880; but we failed at that time to obtain any indications 

 of sensitiveness with a reflecting galvanometer. We have since 

 found, however, that when this tellurium spiral is connected in 

 circuit with a galvanic battery and telephone, and exposed to the 

 action of an intermittent beam of sunlight, a distinct musical tone 

 is produced by the telephone. The audible effect is much increased 

 by placing the tellurium cell with the battery in the primary circuit 

 of an induction coil, and placing the telephone in the secondary 

 circuit. 



The enormously high resistance of selenium and the extremely 

 low resistance of tellurium suggested the thought that an alloy of 

 these two substances might possess intermediate electrical properties. 

 We have accordingly mixed together selenium and tellurium in 

 different proportions, and, while we do not feel warranted at the 

 present time in making definite statements concerning the results, 

 I may say that such alloys have proved to be sensitive to the action 

 of light. 



It occurred to Mr. Tainter before my return to Washington last 

 January, that the very great molecular disturbance produced in 

 lamp-black by the action of the intermittent sunlight should pro- 

 duce a corresponding disturbance in an electric current passed 

 through it, in which case lamp-black could be employed in place of 

 selenium in an electrical receiver. This has turned out to be the 

 case, and the importance of the discovery is very great, especially 

 when we consider the expense of such rare substances as selenium 

 and tellurium. 



The form of lamp-black cell we have found most effective is 

 shown in Fig. 8. Silver is deposited upon a plate of glass, and a 

 zigzag line is then scratched through the film, as shown, dividing 

 the silver surface into two portions insulated from one another, 

 having the form of two combs with interlocking teeth. 



Each comb is attached to a screw-cup, so that the cell can be 



* Proc. Roy. Soc, vol. xxiv, p. 163. 



