The Ionization of Atoms by Electron Impact

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Abstract
A method of high precision for the measurement of critical potentials. Precision in critical potential measurements in the past has been seriously limited by the lack of homogeneity in velocities of the electrons. This source of error has been eliminated by separating out magnetically electrons of definite velocities. The electron beams used in the present experiments were not characterized by great homogeneity in velocities but by sharp upper limits to their velocity distributions. Critical potentials were measured as the differences between two retarding potentials—the smallest retarding potential preventing the entrance of the electrons into the Faraday cylinder type of ionization chamber and the largest retarding potential for which the effect under investigation is observed—thereby eliminating errors due to contact electromotive forces.