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When a specially designed field cathode assembly was inserted in a commercial electron microscope which was appropriately modified for the purpose, the geometry of the needle shaped emitter was obtained from micrographs of several of its profiles at various stages of emitter fabrication and experimental use. An investigation of several methods of fabrication revealed that a smooth, simple, and relatively stable tungsten emitter geometry resulted from a refinement of the methods of Benjamin and Jenkins which combines the electrolytic etch and the smoothing action of surface migration. The electric field at any point on an emitter surface was calculated when the emitter geometry was precisely fitted with one equipotential surface from the family surrounding a charged, isolated sphere-on-orthogonal-cone. A theoretical surface distribution of current density for a typical emitter was derived from the calculated field distribution and the wave-mechanical field emission theory. From this result was calculated a value of the emitting area which was in good agreement with experiment.
Dyke et al. (Fri,) studied this question.