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BELL SYSTEM TECHNICAL JOURNAL 



Practical Features of Cells as Used 



The photoelectric cells as used for picture transmission are classified, 

 according to the ah()\e discussion, as central anode, gas filled, colored 

 cells. The shape of the cells is that shown in Figure 1, and also in the 

 photograph, Figure 11, with which is an accompanying scale. The 

 cells are made of pyrex glass, which is chosen because it is highly 

 resistant to corrosion by potassium during the distillation stages. 

 The very long neck of the cells is dictated partly by the space into 

 which the cells are placed in the picture transmission apparatus, in 



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Fig. 11 — Photograph of photoelectric cell of type used in picture transmission. 



part by the desirabilit}' of ha^'ing as long an insulating space as possible. 

 Where, as in earlier t>'pes of photoelectric cells, the alkali metal is in 

 close proximity to the other electrode, leakage currents over the glass 

 surface greatly interfere with accurate results. (In working with ex- 

 tremely small currents it is desirable to have in addition to the consider- 

 able length of glass insulating path, a metallic guard ring in the stem of 

 the cell, which may be earthed.) 



The alkali metal ordinarily used is potassium. This is introduced by 

 distillation on the pump. The cell is first baked to a temperature of 

 400° C. for several hours while on the pump in order to drive out all 

 traces of water vapor. The potassium for use in making up the photo- 

 electric cells is first of all distilled in a vacuum into long glass tubes. 

 In this preliminary distillation, the greater part of the absorbed gaseous 

 impurities are removed. After the cell has been baked out on the 

 pump, a piece of the glass tube containing potassium is broken ofif and 

 introduced into the pump system. Between the point of introduction 

 and the cell are a series of bulbs. The potassium after melting in 

 vacuo is distilled successively through these bulbs and into the photo- 

 electric cell, where it is condensed on the walls of the bulb. A window 

 is then made in the cell by applying a small flame on the appropriate 



