Probability of Ionization of Mercury Vapor by Electron Impact

Electrons from a hot filament were projected into an ionization chamber $A$ containing mercury vapor, and were prevented from scattering by a magnetic field of 400 gauss parallel to the electron beam. The positive ions produced in a definite length of $A$ were collected on a plate electrode while the primary electrons after passing through $A$ were collected by a special form of Faraday cylinder. The resulting curve for $N$, the number of positive charges produced in 1 cm path by an electron moving through the vapor at 1 mm pressure as a function of the electron energy in volts is given. This curve, in good agreement with that obtained by Compton and Van Voorhis for mercury vapor, shows a maximum value of 20.5 at 90 volts. The values of $P$, the average number of positive charges formed at an impact, were then calculated from $N$ by the relation $P=N\lambda$ where $\lambda$ is the electronic mean free path. Assuming the kinetic theory value for $\lambda$ the curve for $P$ shows a maximum value of 0.35 at 90 volts, agreeing well with the value 0.32 at 100 volts calculated similarly by Compton and Van Voorhis. However, when the values of $\lambda$ obtained by direct experiment are employed, the curve for $P$ shows no indication of reaching a maximum. At 400 volts, $P$ has a value of about 1 and the curve indicates values greater than 1 at higher voltages. Since the probability of ionization at an impact cannot be greater than unity, the assumptions made in evaluating this quantity have been examined more closely.