334 BELL SYSTEM TECHNICAL JOURNAL 



passed from a high voltage source, until, by illuminating the alkali 

 metal surface and reading the current on a sensitive galvanometer, it 

 is found that a maximum of sensitiveness has been attained. The 

 hydrogen is then completely removed by long continued pumping. 

 The final step in the preparation of the cell consists in the introduction 

 of a small quantity of carefully purified argon. The argon for this 

 purpose is held in a reservoir in which there is a pool of sodium- 

 potassium alloy. By passing an electric discharge from this pool to an 

 electrode through the gas, the argon is purified of all active impurities. 

 It is introduced into the cell through the same series of potassium 

 coated bulbs already mentioned, the potassium in the meantime having 

 been vigorously heated to drive off all occluded hydrogen, so that the 

 gas when it finally reaches the photoelectric cell is entirely inert. The 

 gas pressure is carefully adjusted while the cell is still on the pump so 

 as to give an optimum effect, after which the cell is sealed off. 



Typical voltage-current characteristics of the cells thus made are as 

 shown on Figure 12, where the currents indicated are those obtained 

 from an illumination of 100 meter candles from a vacuum tungsten 

 lamp, the aperture of the cell being 1.5 sq. cm. It will be noted that, 

 unlike the ideal characteristic shown in Figure 6, the actual cells shows 

 a small current in the opposite direction for positive voltages applied 

 to the sensitive surface. This is because practically it is very difficult 

 to prevent some alkali metal from depositing on the anode, which 

 thus becomes light sensitive, and responds to the scattered and re- 

 flected light in the cell. 



For use in the picture transmission apparatus, the cells are mounted 

 in tubular metal cases, from which they are insulated by hard rubber 

 rings attached to the glass stem, by sealing wax. The cells in their 

 cases are handled as units; and are sufficiently rugged to be readily 

 shipped from place to place. Their characteristics remain practically 

 unchanged indefinitely. 



SELECTED BIBLIOGRAPHY 



Books 



Photo-electricity, H. Stanley x^Uen, 2nd Ed. 1925 (containing bibliography of 

 articles up to 1925). 



Photo-electricity, A. L. Hughes, 1914. 



Die Lichtelektrische Erscheinungen, Pohl and Pringsheim, 1914. 



National Research Council Report on Photo-electricity, A. L. Hughes, 1921. 



Recent Articles 



The Transmission of Pictures over Telephone Lines, H. E. Ives, J. W. Horton, 

 R. D. Parker and A. B. Clark, Bell System Technical Journal, April, 1925. 



