The Mobility of Ions in Air. Part IV. Investigations by Two New Methods

In a previous communication to the Royal Society (vol. 110, p. 342 (1926)) an account was given of measurements of mobility in air containing water vapour. The essential feature of the method used was the production of a layer of ions by a momentary flash of α -rays and their subjection to a special type of alternating field. When all the ions of one sign are of the same mobility the current is confined to a narrow range of frequency of alternation of the field, rising to a sharp maximum or “peak” at a frequency which is a measure of the mobility. The apparatus was used to investigate the effect of water and also of a series of organic vapours on the mobility of air ions, and, incidentally, gave further evidence of the change which Erikson discovered in the nature of the positive ions with age. In considering the extension of the experiments to other problems, certain disadvantages of the method were apparent. Firstly, to obtain the necessary flashes of α -rays in synchronism with the alternating field, the polonium source was mounted outside the ionisation chamber, and the a-rays admitted through a thin mica window. The apparatus, therefore, could not be made thoroughly airtight, and was not suitable for extremely dry air or for other gases. Secondly, the age of the ions was entirely governed by the time taken by them in crossing the mobility box. Therefore, not only was the age strictly limited, but also in the case of positive ions there was definite evidence of a transformation in the nature of some of the ions during the measurement itself.