Sound by Radiant Energy. 521 



nium, 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 inter- 

 mittent 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 ex- 

 tremely low resistance of tellurium suggested the thought 

 that an alloy of these two substances might possess inter- 

 mediate 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 pro- 

 duced in lampblack by the action of intermittent sunlight 

 should produce a corresponding disturbance in an electric 

 current passed through it, in which case lampblack could be 

 employed in place of selenium in an electrical receiver. This 

 has turned out to be the case ; and the importance of the dis- 

 covery is very great, especially when we consider the expense 

 of such rare substances as selenium and tellurium. 



The form of lampblack-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 inter- 

 locking teeth. 



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

 be placed in an electrical circuit when required. The surface 

 is then smoked until a good film of lampblack is obtained, 

 filling the interstices between the teeth of the silver combs. 

 When the lampblack-cell is connected with a telephone and 

 galvanic battery, and exposed to the influence of an inter- 

 mittent beam of sunlight, a loud musical tone is produced by 

 the telephone. This result seems to be due rather to the 

 physical condition than to the nature of the conducting 

 material employed, as metals in a spongy condition produce 



