Ionization Currents in Divergent Fields in Hydrogen and in Air

Ionization currents were measured in hydrogen and in dry, mercury-free air between concentric cylinders for pressures from 0.01 mm to 760 mm. Above the pressures corresponding to the minimum sparking potentials, $\left(\frac{1}{r}\right)\left(\frac{1}{p}\right) log\frac{i}{i_0}$ was a single-valued, continuous function of ${\left(\frac{X}{p}\right)}_{max}$. Calculated values of apparent $\alpha$ for air agreed with uniform field values at low $\frac{X}{p}$ but were high at higher $\frac{X}{p}$. In hydrogen apparent $\alpha$ was high over the full range of variables studied. Auxiliary anodes permitted estimates of the location of the ionization which was found to agree with the spatial distribution predicted by Morton. Electrons released in the low and collected in the high field produced the same ionization as for the reverse case except at low pressure in hydrogen where about 10 percent less ionization occurred for electrons released in the low field. The peaks of Morton's $\frac{i}{i_0} \mathrm{vs}.\mathrm{} p$ curves were identified with the minimum sparking potential. Application of the back diffusion equation of Rice permitted the evaluation of $i_0$ at higher pressures than hitherto has been possible. Comparison of the present results with those reported by Fisher and Weissler show that the present results can be directly applied if the focal length of the point parabola is substituted for $r$ in the electron multiplication parameter. On the basis of this agreement, it is expected that the present values of the parameter may be applied to any geometry in which the field varies inversely as the distance from a fixed point. A necessary condition for the application of the observed values of the parameter to any system is that the electrons give up all of their ionizing energy to the gas before being collected.