THE ALKALI METAL PHOTOELECTRIC CELL 



331 



process, probably because of their lower melting points, and hence 

 cells made of potassium may represent practically the best performance 

 which is now attainable. 



Effect of Temperature 



It has long been held that the photoelectric effect is independent of 

 temperature. Recent experiments, however, have shown that the 

 alkali metals are affected in their photoelectric response by variation 

 of temperature. A typical set of data for potassium is shown in 

 Figure 10. It will be noted that the influence of temperature is small 



-200° -100° 0* *I00*' 



Fig. 10 — Variation of photoelectric sensitiveness of potassium with temperature. 



for the shorter wave lengths of light, but considerable for long wave 

 excitation. These data were secured in highly exhausted cells of 

 pure alkali metal. When gas is present or a large amount of alkali 

 metal vapor can deposit on the cooled surface, the effect of decreased 

 temperature may be to increase the photoelectric current. It will be 

 noted that the changes shown in Figure 10 are insignificant over the 

 ordinates range of room temperatures, and for practical purposes, 

 particularly for picture transmission, the effects of temperature on 

 the performance of photoelectric cells may be taken as negligible. 



