The dependence of the mobility of ions in air on the relative humidity

The relationship between relative humidity and the mobility of negative ions in air at atmospheric pressure has been examined with a view to possible applications in hygrometric measurements. The method employed was a modification of Zeleny's original method, the end of a wind channel being closed by a disc of gauze fitted with a guard ring through which a steady stream of air of definite humidity was pumped. The motion of the negative ions due to the action of the air stream was balanced by a counter potential gradient, and the mobility was deduced from the critical potential required to produce a balance. The rate of air flow was measured by means of a Ewing ball and tube flowmeter, a hollow glass sphere being used to make it suitable for low air rates. Efforts were made to construct a direct indicating instrument by amplifying the current sufficiently to obtain deflections on a robust moving-coil indicator, but the variations in the amplification factor were found to be too serious.