Air Ion Mobility Research Articles

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.

The mobility of ions in air. Part.V―The transformation of positive ions at short ages

It was shown in Part IV of this paper that both the methods there described, which involved the use of closed vessels, gave variable results for the rate of transformation of positive ions, and values of the rate which were higher than those obtained by Erikson or by Tyndall and Grindley. It was felt that this might be due in part at least to products of chemical action arising from the α -rays, or to the impurities given off from the wails which would accumulate in a closed vessel unless some agent is present to remove them. To investigate this possibility further, the writers set up an air-blast method in which the effect of adding or taking away such impurities could be conveniently studied.

The Effect of Water Vapor on the Mobility of Gaseous Ions in Air

In this paper additional results are given which indicate that an ${\mathrm{H}}_{2}$O molecule gives up an electron to the final positive air ion and thus forms an ${\mathrm{H}}_{2}$${\mathrm{O}}^{+}$ ion of a greater mobility. Results are also given showing that the reciprocal of the mobility bears a linear relationship to the humidity.

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.

The Mobility of Acetylene Ions in Air

The mobilities in air of the positive and negative ions produced in acetylene by means of the alpha-rays from polonium have been measured by means of a blast method. It is found that in acetylene, as in air, carbon dioxide, hydrogen, and argon, only one negative ion is formed. This ion has the same mobility in air as have the negative ions formed in the other gases referred to. On the other hand, the positive ion formed in acetylene differs from those formed in the other gases in that it does not, at least up to an age of one second, change over into a more stable ion of smaller mobility. The positive ion of acetylene has a mobility slightly less than that of the negative ion and in this respect resembles the initial positive ion of hydrogen. The initial positive ions in air, carbon dioxide and argon have the same mobility as have the negative ions of these gases. It is also found that a neutral acetylene molecule is able to take up the charge of the final positive ion formed in air and in nitrogen. The resulting positive ion has the same mobility in air as the normal positive acetylene ion. This indicates that the process is of the nature of an electron interchange rather than a chemical oxidation effect.

The mobility of ions in air. Part III.—Air containing organic vapours

The data on mobility of gaseous ions available at present have not led to a satisfactory quantitative theory of ionic structure because of the large number of variables necessarily entering into the formulæ deduced. The writers feel that the most fruitful line of experimental attack is to carry out a series of different experiments under conditions chosen so that one or more of these variables may be regarded, at any rate approximately, as unchanged throughout the series. The work herein described is an attempt in that direction and deals with the mobility of ions in air containing the vapours of a series of closely related organic substances. It is well known that the presence of water vapour in air causes a reduction in the mobility of the negative ions. The effect has recently been studied in detail from small traces up to saturation by one of the writers (A. M. T.) and Mr. G. C. Grindley. Other vapours are also known to be similarly effective. Thus Rutherford showed that alcohol and ether vapour lower the mobility of negative ions in air. Przibram found that various saturated organic vapours affect both positive and negative mobilities, though quantitatively his method is not free from objection. Wellisch found that 10 mm. of alcohol or acetone introduced into air at atmospheric pressure caused a reduction in both mobilities, but that 6 mm. of ethyl bromide or iodide had no effect.

Societies and Academies

Societies and Academies

The Mobility of Argon and Hydrogen Ions in Air

The Mobility of Argon and Hydrogen Ions in Air

The absolute mobilities of negative ions in air

The absolute mobilities of negative ions in air

The Formation of Negative Ions in Air.

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.

Mobilities of Ions in Air, Hydrogen, and Nitrogen

Mobilities of Ions in Air, Hydrogen, and Nitrogen

LI. On the mobilities of ions in air at high pressures

LI. <i>On the mobilities of ions in air at high pressures</i>

Mobilities of ions in air

Mobilities of ions in air

On The Mobility of Ions in Air at High Pressures

Received 20 October 1910DOI:https://doi.org/10.1103/PhysRevSeriesI.34.53©1912 American Physical Society