The Effect of a Hydrogen Atmosphere on the Velocity Distribution Among Thermionic Electrons

Abstract

The thermionic current from a tungsten or platinum filament to a coaxial cylindrical electrode was measured for different retarding potentials first in vacuum and then in hydrogen keeping the temperature of the filament constant. The heating current was made intermittent to eliminate the magnetic field and the potential drop along the filament when the thermionic current was being measured. The filament temperature was kept constant to $\frac{1}{10}$ percent by a Wheatstone bridge arrangement of which the filament was made one arm. Filament temperatures ranging from 1520\ifmmode^\circ\else\textdegree\fi{}K to 1990\ifmmode^\circ\else\textdegree\fi{}K, and pressures of hydrogen from 0 to 0.25 mm Hg were used. The distribution of velocities among the electrons in vacuum and in hydrogen was found to follow Maxwell's law; and contrary to the experience of former observers the temperature calculated from the Maxwellian distribution was found to be the same for the case of a vacuum as for that where hydrogen was present to an accuracy higher than 0.7 percent which is within the accuracy of the experimental data. Moreover, the temperature so calculated was for all cases in close agreement with the filament temperature estimated from data of Forsythe and Worthing for tungsten and those of Pirrani for platinum. The maintenance of thermal equilibrium between the electrons and the filament in hydrogen suggests the elastic nature of the collisions between the electronsand the hydrogen molecules.