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The effect of space charge in. magnetrons is found to have a profound effect upon performance even though the critical motions of the electrons may be correctly determined by disregarding it. The inclusion of space charge gives a simple picture based on a rotating cloud of space charge. Possible vibration modes for this space-charge cloud are discussed and their relation to the impedance seen from an external circuit is correlated with the possibility of oscillation production. For obtaining mathematical solutions, the approximation of assuming the filament cylinder to be infinitely small in diameter has been necessary. Places where this assumption causes trouble are pointed out. Finally, the opposite case of a very large filament diameter is discussed briefly in terms of a planar structure. The description of the results is visualized by comparison with well-known problems of electrotechnics, such as alternators or cyclotrons.