Abstract

This paper is a tutorial in nature. It is an attempt to explain the operation of a thermionic electron gun in a way that is more easily understood than the accounts given in standard texts. In particular, it addresses two questions. The saturation of a self-biased gun is shown to be the result of the reduction of the emitting area. The loss of structure in the image of the sources arises from the fact that, at the saturation point, the area of emissions is small.