Non-Existence of Ion Mobility Spectrum in Air

A modified form of the Rutherford a.c. method of measuring ion speeds, as suggested by Loeb and Bradbury, has been used to investigate the range of speeds with which negative ions in air travel. It differs in that the ions are produced only during a small fraction of the a.c. period, this being accomplished by interrupting the ultraviolet light beam with a rotating shutter. The theoretical curves of d(ii₀)dV against V assuming both the presence and the absence of an ion spectrum are given. The experimental curves for ions 0.03 to 0.006 sec. old agree closely with the theoretical curves for a spectrum, but it is shown that on application of a diffusion correction the spectrum in clean dry air reduces to negligible width; the presence of ozone in considerable amounts, however, produces a spectrum extending 10 to 12 percent on each side of the mean mobility.