PRODTJCTION OF SOUND BY RADIANT ENERGY. 325 



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 tel- 

 lurium 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 projDerties. 

 We have accordingly mixed together selenium and tellurium in differ- 

 ent 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 intermittent sunlight should produce a corre- 

 sponding disturbance in an electric current passed through it, in which 

 case lampblack could be employed in place 

 of selenium in an electrical receiver. This 

 haa turned out to be the case, and the im- 

 portance of the discovery 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 sur- 

 face into two portions insulated from one 

 another, having the form of two combs 



with interlockino^ teeth. 



Each comb is attached to a screw-cup, 

 so that the cell can be placed in an elec- 

 trical circuit when required. The surface 

 is then smoked until a good film of lamp- 

 black is obtained, filling the interstices be- 

 tween the teeth of the silver combs. When 

 the lampblack cell is connected with a tele- 

 phone and galvanic battery, and exposed 

 to the influence of an intermittent 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 similar effects. For 



