No. 7.] BELL — THE PHOTOPHONE. 403 



changes ia the vibratory movement of a particle of air duriug 

 the transmission of a sound of definite quality through the 

 atmosphere. The curve that would graphically represent the 

 changes of light would be similar in shape to that representing 

 the movement of the air. I do not know wiicther this conceptiou 

 had been clearly realized by " J. F. W.," of Kew, or by Mr. 

 Sargent, of Philadelphia; but to Mr. D.ivid Brown of London, 

 ia undoubtedly due the honor of having distinctly and independ- 

 ently formulated the conception, and of having d<:'vised appar- 

 atus — though of a crude nature — for carrying it i 'to execution. 

 It is greatly due to the genius and perseverance of my friend, 

 Mr. Sumner Tainter, of Watertown, Mass., that the problem of 

 producing and reproducing sound by the agency of light has at 

 last been successfully solved. 



The first point to which we devoted our attention was the 

 reduction of the resistance of crystalline selenium within manage- 

 able limits. The resistance of selenium cells employed by former 

 experimenters was measured in millions of ohms, and we do not 

 know of any record of a selenium cell measuring less than 250,000 

 ohms in the dark. We hive succeeded in producing sensitive 

 selenium cells measuring only 300 ohms in the dark, and 155 

 olims in the light. All former experimenters seemed to have used 

 platinum for the conducting pirt of their selenium cells, excepting 

 Werner Siemens, who found that iron and copper might be em- 

 ployed. We have also discovered that brass, although chemically 

 acted upon by selenium, forms an excellent and convenient 

 material ; indeed, we are inclined to believe that the chemical 

 action between the brass and selenium has contributed to the low 

 resistance of our cells by forming an intimate bond of union be- 

 tween the selenium and brass. We have observed that melted 

 selenium behaves to the other substances as water to a greasy 

 surface, and we are inclined to think that when selenium is used in 

 connection with metals not chemically acted upon by it, the points 

 of contact between selenium and the metal offer a considerable 

 amount of resistance to the passage of a galvanic current. By 

 using brass we have been enabled to construct a large number of 

 selenium cells of different forms. The mode of applying the sele- 

 nium is as follows : The cell is heated, and, when hot enough, a 

 stick of selenium is rubbed over the surface. In order to acquire 

 conductivity and sensitiveness, the selenium must next undergo 

 a process of annealing. 



