ELECTRIC CHARGING OF ELECTRON-MICROSCOPE SPECIMENS

In electron microscopy, and particularly in small-angle electron diffraction, the electric charging of thin insulating films, which are frequently used to support specimens, has been regarded as a nuisance and techniques have been developed to minimize it. One particular phenomenon, however, has been of scientific interest, and this is the 'fluctuating granularity' observed in some shadow-projection experiments. In the main previous authors have suggested some kind of electrical break-down as a mechanism of this effect, but in the present paper it is shown that a statistical theory of space charge can account quantitatively for this so-called 'bee-swarm' effect, and that the apparent size of the 'grains' is a function of the conditions of observation.