Thermionic effects similar to those studied by Langmuir and Kingdon for caesium have been found for rubidium and potassium. Because of the difference in the electron affinity of tungsten (4.53 volts) and that of an atom of rubidium or potassium (4.16 and 4.32 volts respectively) positive ions are formed which at filament temperatures above 1000° are drawn to the cylindrical collector from the coaxial filament. Below this temperature the ions are adsorbed on the surface of the filament and decrease its work function raising the electron emission. The degree of thermal ionization at various filament temperatures is found to be within the experimental error of that determined from Saha's equation. At low temperatures the filament is partially covered by adsorbed ions and the electron emission increases exponentially with the reciprocal of the temperature. A transition region is reached as the average life of an atom on the surface becomes shorter and then the emission decreases logarithmically with the reciprocal of the temperature. The curves and values are given. The positive ion emission increases logarithmically with the reciprocal of the temperature until a certain fraction of the atoms striking the filament is ionized. Further increase in temperature causes every atom to be ionized. The coincidence of experimentally determined points with the theoretical space charge curve shows each ion to have the mass of a single atom in the range of pressures studied.

Vapor pressure of rubidium and potassium.—The vapor pressure of rubidium and potassium is calculated from the maximum ion current; for rubidium $logp=10.55-\frac{4132}{T}$ and for potassium $logp=11.83-\frac{4964}{T}$; here $p$ is in bars.

• Received 9 February 1926

DOI:https://doi.org/10.1103/PhysRev.27.578