Measurements of the Mobility of Potassium Ions at High Field Intensity and Low Pressure

The mobility of potassium ions in the gases ${\mathrm{H}}_2$, He, ${\mathrm{N}}_2$, and A has been measured by the Townsend method, in which a beam of ions is deflected by a magnetic field. When the mobility is reduced to 760 mm of mercury and 20°C it should be expressible as a function of the ratio of electric intensity $E$ to the pressure $p$. This is true at the highest pressures investigated but not at the lowest, apparently because of a breakdown in the method. At small values of $\frac{E}{p}$ the results agree to within a few percent with the measurements by the Tyndall and Powell method. As $\frac{E}{p}$ is increased, the mobility is at first constant. It then rises, passes through a peak, and finally falls. Pronounced peaks appear in the curves for ${\mathrm{H}}_2$, ${\mathrm{N}}_2$, and A but not for He. The mobility is constant only in weak fields where the ions and molecules interact with a frequency which is independent of the drift velocity of the ions. The rise in mobility with increase in $\frac{E}{p}$ may be interpreted as the result of a decrease in the effectiveness of the forces of attraction between the ions and molecules. As the drift velocity exceeds the thermal velocities of the molecules, the collision frequency becomes greater, and the mobility finally falls. Langevin's theory of mobility may be generalized to express the variation with $\frac{E}{p}$. It is in qualitative agreement with the experimental results, but fails to account for more than a suggestion of the peaks. They are accounted for, however, by a generalization of Hassé and Cook's theory. The experimental data are bracketed by the two theories.