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For over twenty years, in the course of our work on the oxide-coated cathodes, we have frequently come across many phenomena which can not be explained by the semi-conductor theory. This has led us to re-examine carefully most of the parameters, such as work functions, electrical conductivities, spectral absorption, concentration of excess barium, lattice structure,surface energy states, distribution of potentials in the coating, comparsion of thin film emission to coating emission, non-uniformity of emission, flicker noise, instability of impulse current, sparking etc. All these show that the oxide-coated cathode is not an n-type semi-conductor with excess barium as donor. A preliminary theory of surface emission centers which can explain many phenomena in the oxidecoated cathode is proposed.

The present paper is a criticism of the application of the monoatomic layer and simple dipote theory to thermionic emission. We have come to the following conclusions: (1) the existence of the monoatomic layer is conditional, it depends upon the temperature, surface structure and purity of the substrate as well as upon the vacuum environment; (2) the appearance, and sometimes disappearance, of the emission peaks is a problem for further study, and it can not be explained simply by the monoatomic layer and dipole theory; (3) the enhanced emission after adsorption of foreign atoms or molecules is not due to the formation of dipoles which lower the work function of the substrate, but comes from the valence electrons of the adsorbate, and (4) the "L" cathode is a Ba-O-W system, neither monoatomic nor monomolecular, and the dipole theory is not applicable here.