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BELL SYSTEM TECHNICAL JOURNAL 



condition of saturation typical of the high vacuum cell for high nega- 

 tive voltages no longer holds. Instead the photoelectric current is 

 increased by the occurrence of ionization, by collision of the electrons 

 initially produced with the molecules of gas. The current increases 

 with applied voltage in the manner shown in Figure 4, until at some 

 value characteristic of the kind of gas in the cells, the gas breaks down 

 and a visible electrical discharge takes place. The amplifying effect 

 of the gaseous atmosphere increases with the pressure of the gas up 



100 



+V0LT5 ON CATHODE -VQLT5 ON CATHODE 



Fig. 4 — Voltage-current curves for typical central cathode gas filled photoelectric cell. 



to a maximum and then decreases. The value of this optimum pres- 

 sure depends on the kind of gas and the dimensions of the tube. The 

 best pressure is usually a few tenths of a millimeter of mercury. 



As the illumination of the cell is changed, the current changes in 

 exact proportion, that is the illumination-current relationship is rec- 

 tilinear. This relationship holds for both the vacuum and gas cells 

 provided there are no free glass surfaces on which charges may accu- 

 mulate. If the window is made too large it may become charged 

 and cause an appreciable curvature of the illumination-current rela- 

 tionship. 



Central Anode Cells 



In cells with a relativeh- small centrally placed anode, the voltage- 

 current relationship differs from that of the central cathode cells most 

 noticeably in that high applied voltages are necessary in order to 

 insure saturation. Typical voltage current curves for short (Xi) and 

 long (X2) wave length energy, for a central anode cell consisting of a 



