Abstract
The tables associated with the exact Langmuir space charge theory have been represented to a maximum relative error of 0.01% by approximation formulas which are suitable for use with digital computers. Application of the exact theory to a thermionic converter comprising two close‐spaced planar impregnated cathodes has permitted a critical evaluation of the approximate space charge theory of Nottingham. The influence of the electrode separation, the emitter and collector work functions and of the emitter temperature has been investigated. Spectral emittance measurements on a cathode surface at two wavelength has resulted in a more firmly based estimate of the radiation heat transfer between two impregnated cathodes and of the maximum efficiency of an ideal design.