1881.] on Selenium and its appUcaiiona to the Photophoney dc. 531 



21° C* Lot this piece of solouiiim bo gradually raised from a tom- 

 peraturo of zero to a temperature of lUO' C. Wliilo passing from zero 

 to 24:"^, its resistance will rapidly increase. Passing from 24" to 100'^ 

 its resistance will again rapidly diminish. (This experiment was 

 successfully shown.) 



Until Professor Bell directed his attention to selenium, all observa- 

 tions concerning the eftect of light upon its conductivity had been 

 made by means of the galvanometer. But it occurred to him that tho 

 marvellously sensitive telephone which ho has invented might with 

 advantage be used for the j^urposo, and on the 17th May, 1878, ho 

 announced in this theatre "the possibility," to use his own words, "of 

 hearing a shadow by interrupting tho action of light upon selenium." 

 A few days afterwards Mr. Willoughby Smith informed the Society of 

 Telegraph Engineers that he had carried this idea into effect, and had 

 heard the action of a ray of light upon a piece of crystalline selenium. 



When a selenium ceH, a telephone, and a battery are connected in 

 circuit, a uniform current of electricity will, under ordinary circum- 

 stances, flow through the telephone, and a person listening would hear 

 nothing. Suppose now that a series of flashes of light were allowed 

 to fall upon the selenium. In the intervals of darkness the selenium 

 cell would offer a greater resistance to the passage of the electric 

 current than during the intervals of light. The strength of the 

 current would be constantly varying ; and if the flashes succeeded one 

 another quickly enough and with sufficient regularity, a musical note 

 would now be heard by a person listening at the telephone. The 

 exact pitch of this note would of course depend upon the rate at 

 which the flashes succeeded one another, being high when the succes- 

 sion is rapid, low when it is slow. The nature of this sound is very 

 peculiar, reminding one of the moaning of a syren or the rising and 

 falling of the wind. With a sufficiently sensitive cell, powerful 

 battery, and delicate telephone, the sound may be heard at a distance 

 of many feet. 



I shall interrupt the steady beam of light which is now falling 

 upon the cell by causing a zinc disk with radial slits cut in it to 

 rotate in the path of the beam, and the sound produced by the rapid 

 succession of light and shade upon the selenium cell will bo heard in 

 the tele2)hone. When the cell is screened from the light, the sound at 

 once ceases. When the screen is removed, the sound is again heard 

 as before. By using a system analogous to that of dots and dashes, 

 an intermittent beam of light might be employed to convey photo- 

 phonic messages to a distance. 



But Professor Bell has gone further than this. He was not satisfied 

 with merely interrupting a steady beam of light, producing alternately 

 strong light and total darkness, but he aimed at graduating its 



* The experiment has since been repeated with live other cells, and their 

 temperatures of maximum resiatauco were foimd to be 23°, l-i°, 30°, 25°, aud 22° 

 respectively. 



