C^SIUM-OXYGEN-SILVER PHOTOELECTRIC CELL 335 



corresponds with the wave-length of the first line of the principal 

 series of the atomic spectrum of the alkali metal. 



The first composite surface to attain practical importance was the 

 so-called potassium hydride cell discovered by Elster and Geitel ^ in 

 which a potassium surface is sensitized by a glow discharge in hydrogen. 

 This cell has its maximum response at 4,350 A. and its photoelectric 

 threshold at 5,900 A. The response of a good potassium hydride 

 (vacuum) cell to a light source at a color temperature of 2,710° K. 

 is about one microampere per lumen, which is eighty times that 

 obtainable with pure potassium surfaces. 



This sensitivity is still below that required for many technological 

 applications. Also, the potassium hydride surface is unstable even 

 at ordinary temperatures and may deteriorate rapidly in use or in 

 storage. So there has been a great demand in engineering applications 

 for both a more stable device, and a cell more sensitive to the red 

 and infra-red light which constitutes the major part of the emission 

 from common incandescent light sources. 



The photoelectric threshold is a direct measure of the work necessary 

 to liberate an electron from a surface, i.e., the "work function" 

 which also figures in the thermionic effect. That is, both red sensitive 

 photoelectric cells and active thermionic filaments possess low values 

 of the work function. The study of the thermionic effect in adsorbed 

 films of caesium on tungsten and on oxidized tungsten by Langmuir 

 and Kingdon ^ and by Becker ^ have indicated surprisingly low values 

 of the electron work function. It was to be expected that some 

 similar surface should possess a high order of photoelectric response to 

 red light. Research along these lines has resulted in this and other 

 laboratories in the development of the caesium-oxygen-silver photo- 

 electric cell. Early work on cells of this type is reported by Koller ^ 

 and by Campbell.'' 



For the cells discussed in this paper the active photoelectric surface 

 is formed on a roughened silver sheet. This is slightly oxidized by 

 making it the cathode in a glow discharge in oxygen. Caesium is then 

 generated by chemical reaction in a pellet enclosed within the photo- 

 electric cell bulb. Finally, by a proper temperature cycle the caesium 

 is condensed on the silver oxide surface of the cathode and allowed 

 to react with it to form the active photoelectric surface. 



3 Elster and Geitel, Phys. Zeit., 11, 257 (1910). 

 * Langmuir and Kingdon, Phys. Rev., 21, 380 (1923) abstr. 

 K. H. Kingdon, Phys. Rev., 24, 510 (1924). 

 Langmuir and Kingdon, Proc. Roy. Soc, 107-A, 61 (1925). 

 "J. A. Becker, Phys. Rev., 28, 341 (1926). 



8 L. R. Koller, Phys. Rev., 33, 1082 (1929) abstr. Phys. Rev., 36, 1639 (1930). 

 ^N. R. Campbell, Phil. Mag., 12, 173 (1931). "Photoelectric Cells and Their 

 Applications" (Phys. Soc. London, 1930), p. 10. 



