386 BEI.L SYSTEM TECHXICAL JOURNAL 



plane glass surface shows the normal and selective effects less well 

 differentiated than for the similar coatings which form on metal 

 plates. A rubidium film formed on the inside of a glass cylinder is 

 found to exhibit, in the dark, a pure ohmic resistance. This decreases 

 under illumination in a manner which appears to be explained as 

 due to the liberation of photo-electrons which under a potential 

 gradient form an added current along the tube. 



The Influence of Temperature on the Photo-Electric Effect of the 

 Alkali Metals} Herbert E. Ives and A. L. Johnsrud. Special 

 cells having a hollow central cathode w^ere immersed in liquid air for 

 an extended period to insure that any gases, if present, were con- 

 densed on the outer alkali metal coated walls. The temperature 

 of the cathode was controlled by a stream of evaporating liquid air, 

 whereby all temperatures between +20 and -180° C. could be 

 attained and held constant and be measured. In these cells the 

 variation of photoelectric current with temperature in sodium, po- 

 tassium, and rubidium is continuous, without abrupt changes. The 

 effect is relatively small for sodium, showing hardly at all for blue 

 light or white light, but clearly for yellow light. The behavior of 

 rubidium is similar to that pre\-iously reported for potassium. 



In a second form of cell, potassium was collected in a deep pool. 

 By slowly cooling the metal from the molten condition, smooth 

 crystalline surfaces were obtained. With these annealed potassium 

 surfaces, the variation of photoelectric current with temperature is 

 represented by curves varying systematically in shape with the color 

 of the light, and the effect is far greater than previously reported, 

 amounting, for yellow light, to a variation of 10 to 15 times between 

 room and liquid air temperature. When the surface is roughened 

 curves of the previously reported type are obtained. Small pools 

 give erratic effects, showing changes in opposite directions for differ- 

 ent portions of the temperature range. It is concluded that the varia- 

 tion of photoelectric effect is intimately connected with the strains 

 produced in the surface by expansion and contraction with tem- 

 perature. 



Positive Rays in Thermionic Vacuum Tubes.} Herbert E. Ives. 

 Thermionic tubes in which a quantity of alkali metal is present ex- 

 hibit not only the normal electron current from the heated filament, 

 but a positive current, which at low filament temperatures may be 



^Journal of the Optical Society of America & Review of Scientific Instruments, 

 Vol. 11, No. 6. pp. 565-579, Dec, 1925. 



* Journal of the Franklin Institute, Vol. 201, pp. 47-69, Jan., 1926. 



