The Alkali Metal Photoelectric Cell 



By HERBERT E. IVES 



Introduction 



IN the development of the commercial system of picture trans- 

 mission now in operation over certain of the Bell System lines, 

 one of the initial problems was the choice of a method of transforming 

 the light and shade of the picture to be transmitted into properties of 

 an electric current. There are in general two methods of accomplishing 

 this. The first.whichwemaytermthephoto-mechanicalmethod, utilizes 

 some photographic process to produce a mechanical structure, which 

 may be used either to make and break contact, or to produce mechan- 

 ical movement of some element whose motion produces a variable 

 electric current. The second method consists in the utilization of 

 some light sensitive device which produces or varies an electric current. 



An indispensable requirement in the electrical transmission of 

 pictures is speed in conveying the picture from one point to another. 

 The choice of a method of transforming light and shade into an elec- 

 trical current will therefore, other things being equal, be that method 

 which requires the least time for the transformation. It is on this 

 basis that the photo-mechanical methods were not favorably con- 

 sidered in this development. The preparation of the line or dot 

 structure image, similar to the half tone plate, or the preparation of a 

 photo-relief, are processes which cannot be completed in less than one 

 to two hours, and involve a delay which in many cases would seriously 

 detract from the advantages of electrical transmission over other 

 means now available, such as the airplane. 



In choosing a photo-sensitive device for this purpose, certain re- 

 quirements had to be met. The light responsive device should be as 

 nearly as possible instantaneous in its action. The response should 

 also be proportional to the light intensity. These requirements cannot 

 be met by any photo-sensitive devices of the group whose resistance 

 changes under the action of light, such as selenium. The field was 

 therefore limited to the photoelectric cell, of the type in which the effect 

 of light is to release electrons from the surface of the light sensitive 

 element and so cause an electric current to flow in the space between 

 the light sensitive surface and another electrode. Photoelectric cells 

 are considerably less sensitive than the best variable photo-resistances, 

 but while this characteristic would have made them difficult to utilize 

 in the earlier days of efforts at picture transmission, the development 

 of vacuum tube amplifiers admirably fitted for amplifying photo- 



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