Using a self‐biased electron gun the current arriving at the specimen, the current density at the specimen, and the area of illumination of the specimen have been determined for different values of the filament height. The variation of current density at the specimen with angular aperture of illumination has been determined and the results compared with those for the zero‐biased gun. The performance of the self‐biased gun is assessed and some of the factors involved in its operation and maintenance are discussed.