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A new and simple electron microscope has been designed for small cylindrical filaments. Die marks are found to be prominent in the electron images of all drawn wires. The activation of thoriated tungsten has been studied. Electron emission from single crystals grown in drawn pure tungsten filaments varies around the wire circumference and depends systematically on crystallographic direction. This variation of emission with crystallographic direction is more pronounced and complex when the filament is allowed to self-activate in cesium or potassium vapor.
Johnson et al. (1936) studied this question.