Experimental Evidence for the Essential Identity of the Selective and Normal Photo-Electric Effects.

In the fall of 1913, while studying the photo-electric properties of freshly cut surfaces of the alkali metals in extreme vacua, we observed that immediately after first cutting, the fresh surface of sodium showed very large photo-sensitiveness when tested with monochromatic light of wave length 5461 A., even when the vacuum was of the order 10-6 mm. as measured by a McLeod gauge. But after several weeks of experimenting and many cuttings a condition was reached in which a freshly cut surface was completely insensitive when illuminated with this wave length. The lost sensitiveness reappeared, however, in the course of not more than two minutes after cutting, and grew rapidly to a very large value in fifteen or twenty minutes. When the gas pressure was of the order of 0.01 mm. the same phenomenon occurred but the rise to a maximum value was less rapid. From these results we began to surmise that photo-electric currents must be due to the influence of some active gas, which diffused from the walls to the metal and whose action upon the surface was retarded by the presence of an inert gas. While we were in the process of testing this hypothesis by repeating the experiment under as diverse conditions as possible the papers of K. appeared nearly simultaneously, all taking the point of view which had been suggested to us by our own experiments, and the last one of them, namely, that of Wiedmann and Hallwachs. asserting with considerable positiveness that the presence of a gas is a necessary condition for the appearance of photo-electric currents in potassium, and that the "photo-electric effect in general takes place in the mixture formed by the body and the absorbed gas." By this time, however, we had obtained data which convinced us that the point of view taken by these authors was untenable and in April, 1914, at the meeting of the American Physical Society, we presented a paper entitled 'Effect of Residual Gases on Contact E. M. F.'s and Photo-currents,'4 in which we described fully and exhibited to the society the curves shown in figure 1. A glance at these curves shows at once that the freshly cut surface is initially strongly photo-sensitive-to lines 2535 and 2804 and that its sensitiveness for these lines diminishes 19