The Alkali Metal Photoelectric Cell

I N the development of the commercial system of picture transmission 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, which we may term thephoto-mechanicalmethod,utilizes some photographic process to produce a mechanical structure, which may be used either to make and break contact, or to produce mechanical 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 electrical 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 considered 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.