The Value of the Townsend Coefficient for Ionization by Collision at Large Plate Distances and Near Atmospheric Pressure

Abstract

The Townsend coefficient of ionization by collision was measured at a pressure of 380 mm in air at plate distances ranging from one to five cm for values of $\frac{X}{p}$ (the ratio of the field strength to the pressure in mm) from 20 to 36.5. It was found that the simple Townsend relation, $i=i_0{e}^{\alpha d}$, is valid at the above pressure and over the above range of plate distances. The ratio $\frac{\alpha }{p}$ of the Townsend coefficient to the pressure in mm was determined as a function of $\frac{X}{p}$ and the least-square reduction shows the function giving closest agreement with the observed data to be $\frac{\alpha }{p}=(2.67\mathrm{}\pm 0.26)\times {10}^{-8\mathrm{}}{\epsilon }^{(0.350\mathrm{}\pm 0.002)\frac{X}{p}}$. From these results it is evident that ionization by collision exists at much lower values of $\frac{X}{p}$ than those given by Paavola. The rise in the saturation photoelectric current at the lowest $\frac{X}{p}$ was compared with the recent work of Bradbury on the nature of the saturation photoelectric current and compares in order of magnitude with values found by extrapolation of the latter's equations. This indicates that below $\frac{X}{p}=20\mathrm{}$ the lack of saturation produces a ficitious value of $\alpha$ which renders measurements of $\alpha$ below $\frac{X}{p}=20\mathrm{}$ of doubtful value. No abnormal increase in the current with plate distance was found even within two percent of sparking. This indicates that if ionization by positive ions does exist it must occur at fields closer to sparking than two percent.