EXHIBITION OF ELECTRICITY AT PARIS. 



257 



given to the world within the last few years, 

 none seem more marvelous than the photo- 

 phone of Professor Graham Bell. This is a 

 telephone in which a beam of light is made to 

 take the place of the metallic conductor. Va- 

 riations in tlie intensity of a ray of light, pro- 

 duced by the vibrations of a flexible mirror, or 

 of an elastic membrane with an aperture for 

 the passage of the light, under the influence of 

 sound-vibrations, are made to set up in a tele- 

 phone wire the same electric vibrations as are 

 conveyed over an ordinary continuous tele- 

 phonic conductor, and cause the resonant re- 

 ceiver to reproduce the sound-waves. This 

 takes place through the medium of a substance 

 possessing the curious property of conducting 

 electricity under the influence of light, but re- 

 sisting the passage of the current in the dark. 

 This mysterious phenomenon was observed in 

 the rare element selenium, belonging to the 

 sulphur group, by Willoughby Smith, in 1873. 

 The conductivity of selenium varies exactly in 

 proportion to the intensity of the light to 

 which it is exposed. Bell found that a piece 

 of selenium, acted upon by a continuous gal- 

 vanic current, and placed in a telephonic cir- 

 cuit, when subjected to rapid alternations of 

 light and shade, produced by a revolving mir- 

 ror, would cause the telephone receiver to give 

 forth musical tones. Thus, the action of vari- 

 able light, through the peculiar sensitiveness of 

 selenium, produced the molecular vibrations in 

 tlie electric conductor which generate sound 

 in the receiver. The note sounded was the 



one whose vibrations, per second, is equal to 

 the number of revolutions made by the mirror. 

 This means, by which light, through its varia- 

 tions in intensity, gives rise to sonorous vibra- 

 tions, was utilized to transmit the effects of 

 sounds, as human speech, and to reproduce the 

 sounds at a distance in the following manner: 

 VOL. xxi. 17 A 



An aperture in a flexible membrane was placed 

 opposite an aperture in a parallel fixed plate. 

 Through these two holes a ray of strong light 

 was passed, which was concentrated at the 

 farther end by a concave mirror upon a pencil 

 of selenium, connected with the receiver of a 

 speaking-telephone, and subjected to a con- 

 tinuous current from a galvanic battery. The 

 sound of the voice would produce vibrations in 

 the membrane, whose amplitude and rapidityl 

 would cause corresponding variations in the 

 intensity of the ray passing through the two 

 apertures. The effects of these variations on 

 the selenium gave rise to corresponding undu- 

 lations in tlie electric current, which were re- 

 converted into sound- vibrations by the receiver. 

 A better mode of affecting the luminous ray by 

 the sound-waves was found to be to reflect the 

 ray to the receiving apparatus with a plane flex- 

 ible mirror of silvered mica, or exceedingly thin 

 glass, which, responding to the sound-vibrations 

 of the voice, modified by its movements the in- 

 tensity of the reflected ray. Electric light, as 

 well as sunlight, and even gas- and lamp-light, 

 have been found capable of conducting the 

 sound-generating undulations. 



Bell's and Tainter's experiences of the reso- 

 nance of various substances under the action 

 of intermittent light (see RADIOPIIONY) sug- 

 gested the trial of the most sensitive of these 

 substances in a photophonic receiver. Lamp- 

 black, by far the most powerful generator of 

 sound under the influence of the intermittent 

 beam, was accordingly tried, and gave decided 

 results, without the intermediation of the elec- 

 tric current. A photophonic transmitter, with 

 a diaphragm only five centimetres in diam- 

 eter, reflected a beam received directly from 

 the sun to a smoked receiver, which, at a dis- 

 tance of forty metres, reproduced distinctly 

 words spoken into the transmitter in a low 

 voice. 



Dr. James Moser has shown that the action 

 of a selenium photophone is that of a micro- 

 phone, and is due to the slight and imperfect 

 contact in the selenium-cell, and the changes 

 in resistance are caused by alterations in the 

 degree of contact effected by the absorbed rays 

 of the intermittent beam. These changes, he 

 maintains, are due to the heating and chemical 

 effects of the illuminating rays absorbed. 



In telephones proper few changes have been 

 made in the instruments as originally patented 

 by Bell, Gray, and Edison. Bell's telephone 

 consists of a straight magnet, with one pole 

 wrapped with a coil of insulated wire, and an 

 iron diaphragm in front of it for receiving or 

 transmitting the sounds. In later forms, both 

 poles of the magnet are made to act upon the 

 diaphragm. The remarkable invention of the 

 microphone, by Hughes, has greatly increased 

 the utility of the telephone. Hughes discovered 

 that, when pieces of carbon in loose contact, 

 resting upon a piece of wood, are placed in the 

 circuit of a telephone, the very slightest vibra- 

 tions communicated to the wood are heard dis- 



