The main effort in the work described has been directed towards the measurement of the electric field distribution in a cut‐off magnetron, based on the electron optical shadow method. The space charge distribution can be determined qualitatively from the measured electric field distribution. The first objective is the development of a convenient method for such a study. In this work an elementary magnetron was investigated by sending an electron beam axially through the tube.The conclusions of the study are that the space charge distribution does not conform to Hull's classical theory (which predicts that the space charge density is approximately constant out to a well‐defined radius and zero beyond this cloud radius). Three different space charge distributions have been observed. One distribution has a sharp maximum near the cloud radius. Another falls off gradually toward the cathode; a third is time‐variant. The shape of the space charge distribution is closely related to the symmetry of the magnetron.