The thermionic emission from a spherical tungsten single crystal has been observed. Photographs and diagrams show qualitatively the dependence of this emission on crystallographic direction. Nearly all the maxima and minima lie on a 110 zone. Similar observations have been made on the same crystal when caesium and barium were adsorbed thereon. Adsorption forces are largest for caesium on surfaces of highest work function. For barium the adsorption forces appear to be more dependent on surface structure as the force between the ion and its image contributes the major part of the adsorption energy. The behavior of adsorption as a function of crystallographic direction is such that the sphere surface may be approximated by that which would be obtained by carving the sphere from a perfect lattice. There is no evidence for a faceted or step-like microstructure. Emission from the spherical crystal when caesium is adsorbing on contaminated complex surfaces is more complicated in its dependence on crystallographic direction than when the surface is clean, and a map of the emission over the crystal changes its configuration with temperature. This behavior is not observed when the surface is clean or only slightly contaminated.

• Received 4 August 1939

DOI:https://doi.org/10.1103/PhysRev.56.947