330 BELL SYSTEM TECHNICAL JOURNAL 



Influence of the Photoelectric Material 

 AND ITS Treatment 



The alkali metals differ in their photoelectric sensitiveness in a per- 

 fectly definite order, which is that of their degree of electro-positive- 

 ness, as shown by their position in the periodic table of the elements. 

 The variation in sensitiveness is correlated with the extension of sen- 

 sitiveness in the spectrum. This progresses regularly from sodium, 

 which in its pure state is not photoelectrically sensitive beyond about 

 .58iu, through potassium and rubidium, to caesium, which is photo- 

 electrically sensitive in the near infra-red. The exact terminations of 

 sensitiveness in the spectrum depend upon the character of the surface 

 and its treatment, and have not been exactly correlated with any 

 other properties of the material. 



In order to attain the greatest sensitiveness with the alkali metals, 

 these are commonly subjected, in the preparation of the photoelectric 

 cell, to what is called the coloring process, discovered by Elster and 

 Geitel. This consists in subjecting the surface to a glow discharge in 

 an atmosphere of hydrogen. The result is to color the otherwise 

 silvery alkali metal a rather deep blue-purple or blue-green. The 

 exact cause of this color is not known, but it has every appearance of 

 being due to the production of small (colloidal) particles of alkali 

 metal. The greater sensitiveness is probably due to the increased 

 effective surface presented by the colloidal particles rather than the 

 increased absorption coefficient of the darker color. Similar colors 

 may be obtained by distilling the alkali metal in a very thin layer on 

 glass and the color of the surface changes when observed by polarized 

 light in much the same manner as do colloidal surfaces of other 

 sorts. 



After the completion of the coloring process, it is necessary to 

 remove all the hydrogen from the cell by pumping. Otherwise the 

 surface will revert to its original uncolored form. In order to obtain 

 the amplifying effect of a gaseous atmosphere, it is customary to intro- 

 duce an inert gas, such as argon or helium, into the cell. 



Cells made in the manner just outlined are reasonably permanent 

 in their important characteristics. Elster and Geitel have made 

 potassium cells in this manner, which when connected with a delicate 

 electrometer exhibited a degree of sensitiveness approximately that of 

 the human eye. According to what has gone before, the most sensitive 

 cells should be obtained if rubidium or caesium are used in place of 

 potassium. It is found however by experiment that rubidium, and 

 particularly caesium, do not lend themselves so well to the coloring 



