RADIO 



287 



reproduces sound except that the sound waves are 

 first changed to electric waves and after having been 

 amplified are made to operate a loud speaker. Study 



a b 



Couilfsy Dell Telephone Laboratories 



FIG. 474. SOUND FILMS 



A. SOUND KECORD ON MARGIN OF PICTURE 



B. SEPARATE SOUND FILM 



Figure 473 and learn how this is accomplished. Figure 

 474 shows a strip of motion picture film with the 

 sound wave photographed beside the picture. 



In reproducing sound 

 which has been placed on the 

 film a modern device called 

 the photo-electric cell is used. 

 This device changes light en- 

 ergy into electrical waves. 

 Figure 475 shows a photo- 

 electric cell. Figure 476 

 shows, above, how sound is 

 recorded on a film and, below, 

 how it is reproduced from the 

 film. Study the diagram care- 

 fully and see if you can ex- 

 plain each of these processes 

 clearly. 



Sending pictures and other 

 documents by wire has been 

 advanced to the point where 

 many of the larger cities of 

 the country are now inter- 



^v.. 



Courtesy Bell Telephone 

 Laboratories 



FIG. 475. PHOTO-ELECTRIC 

 CELL 



connected to furnish this service and many news pic- 

 tures are sent in this way. 



The sending mechanism consists of a cylinder 

 moved by a driven screw. A transparent photograph 

 positive is placed on the cylinder, which is then moved 

 slowly past a very narrow beam of light. Inside the 

 cylinder is a photo-electric cell connected to the re- 



, Film on which 

 sound is recorded 



Light 



I. oud speaker 



Film 



Exciting 

 Lenses '? m P 



FIG. 476. HOW SOUND RECORDED ON FILM IS REPRODUCED 



ceiving device in a distant, city. As the light beam 

 shines through the transparent picture, the lights and 

 shadows recorded on the film are changed to electric 

 waves by the photo-electric cell. After being amplified 

 these electric waves are sent to the receiving device 



DMORAMOF PCTUBE TBANSMnTTNO SYSTEM 



SENDING APPARATUS RtCtlVHO APPARATUS 



FIG. 477. TELEPHOTOGRAPHIC SENDING AND RECEIVING EQUIPMENT 



