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COMMUNICATION 



in the distant city. Figure 477 is a diagram of a tele- 

 photo sending and receiving device. 



The electric waves sent out from the sending set 

 are changed back into light waves at the receiving sta- 



beam by a motor-driven screw. The motor at the send- 

 ing station is driven at exactly the same speed as 

 the one at the receiving station. This is precisely con- 

 trolled by a special circuit. 



Exercise. Study Figure 477 and see if you can trace 

 the process through the sending set and through the re- 

 ceiving set. 



It is possible to send pictures by wireless, in which 

 case about the same type of equipment is used. The 

 electric waves from the photo-electric cell are then 

 sent by wireless instead of by telephone. 



It is possible that within the next few years tele- 

 vision will be perfected to the point where many radio 

 sets will be equipped to receive scenes. At present 

 there are several independent stations broadcasting 

 regular television programs, and it is estimated that 



Courtesy Bell Telephone Laborat 



,_ Courtesy Bell Telephone Laboratories 



FIG. 478. ORIGINAL PHOTOGRAPH (ABOVE) AND TELEPHOTOGRAPHIC 



REPRODUCTION (BELOW) FIG. 480. TELEVISION SENDING DEVICE 



tion by making them control a light valve. The light 

 valve controls a small beam of light from a lamp, per- 

 mitting varying amounts of light to shine through it 

 on to an unexposed negative. The negative is also 

 fastened to a cylinder which is pushed past the light 



Photo-electric cell 



Arc lamp 



HG. 479. HOW SCENES ARE SENT BY WIRELESS 



there are more than fifteen thousand television re-, 

 ceivers in the United States. Recently one of the large 

 chain broadcasting companies began experimentation 

 with television. It is indeed almost unbelievable that 

 in the near future we may sit in our homes and see a 

 play enacted in a distant city. Television can be ac- 

 - complished either over wires or by radio. 



Figure 479 is a diagram of how scenes are sent by 

 television, and Figure 480 is a picture of the equip- 

 ment used in sending. Figure 481 shows the equip- 

 ment used in the reception of television. 



In the sending set a beam of light from an arc lamp 

 is sent through the openings in a rapidly moving disk 

 known as the scanning disk. The openings in the scan- 

 ning disk are arranged in a spiral in such a way that 

 the beam of light scans across the scenes to be sent 

 and is then reflected to the photo-electric cell. The 

 photo-electric cell transforms the light energy that 

 strikes it into electric waves which vary just as the 



