THE ALKALI METAL PHOTOELECTRIC CELL 



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electric currents has remedied this deficiency. A further requirement, 

 that the Hght sensitive device should preferably be sensitive to visible 

 radiation, ruled out the use of those sensitive materials sensitive 

 chiefly to infra-red or ultra-violet radiation. All of these require- 

 ments pointed to the alkali metal photoelectric cell as developed by the 

 work of Elster and Geitel and others. 



General Char-\cteristics of Photoelectric Cells 



The typical photoelectric cell consists of a hermetically sealed glass 

 bulb containing an atmosphere of gas at a low pressure, and provided 

 with two electrodes, one of which is the light sensitive material. In 

 the schematic cell shown in Figure 1, i^ is the photo-sensitive material 



Fig. 1 — Schematic central anode photoelectric cell. 



(cathode), for instance an alkali metal such as potassium, which is 

 spread upon the inside wall of the glass bulb and is connected with 

 the exterior of the bulb by a sealed-in wire, iVu A is the other electrode 

 (anode). As here shown, it is a simple metallic ring connected with 

 a second wire, W2, carried through the stem of the bulb. The two 

 electrodes are shown connected together through a battery, B, and 

 galvanometer, G. The operation of the cell consists in letting light 

 fall upon the cathode through the window, 0. The resulting current 

 may then be measured by the galvanometer, or utilized to operate 

 suitable apparatus. 



A complete study of the photoelectric cell resolves itself into obtain- 

 ing knowledge of the effect of varying a number of factors which enter 

 into its construction and use. Of these we may note: the material 

 which is used for the light sensitive surface, and the treatment to which 

 this material is subjected; the composition and pressure of the gaseous 

 atmosphere; the shape and disposition of the various elements, that 

 is, the structure of the cell. We must investigate the relationship 



