The Formation of Negative Ions in Air.

Abstract

Mobility of Negative Ions in Air at Low Pressures.---The purpose of the experiments was to decide between the two theories of the formation of negative ions, proposed by Wellisch and by J. J. Thomson. The carriers were generated as photo-electrons by focusing ultra-violet light on one plate of a condenser at grazing incidence, thus eliminating stray light effects. Great care was used to secure pure air. The curves obtained agree in general with the results of previous investigators; they show anomalous mobilities below a critical pressure which was found to vary with the frequency of commutation of the E.M.F. No "free electrons" of the Wellisch type were observed. A repetition of the Wellisch experiments with photo-electrons showed that the "free electrons" were really a type of carrier which, started as electrons but attached themselves to molecules in the measuring field, as the Thomson theory demands. To test the Thomson theory, curves were computed assuming certain values for the constants. Theoretical and observed curves are similar in shape; in particular, the inflections near the low voltage end fall fairly closely together; It is also shown that the Wellisch results are in accord with this theory. The chance of negative ion formation constant, $\frac{\mathrm{I}}{n}$, comes out about $\frac{\mathrm{I}}{2.5}\ifmmode\times\else\texttimes\fi{}{\mathrm{IO}}^{5}$ for air. As $\frac{\mathrm{I}}{n}$ is o for nitrogen, and as $\frac{\mathrm{I}}{n}$ in oxygen is about $\frac{\mathrm{I}}{50,000}$, we conclude that it is to the oxygen molecule in air that electrons attach to form ions.