In the development of magnetron oscillators, one of the principal difficulties has been the overheating of the cathode. The present article discusses an effect which is at least partially responsible for this, and which consists in the gaining of excess energy by some electrons, which then bombard the cathode. The effect is of importance also because of its bearing on transit time, orbit shape, tube noise, shape of cutoff curve, etc. Experimental results indicate that a Maxwellian distribution of velocities is superimposed on the orbital velocities, the energy of the random motion being derived from the orbital motion. This leads to the formation of a new type of virtual cathode about the real cathode. The properties of this type of cathode are discussed. Subjects related only indirectly to cathode overheating, but necessary to its understanding, also are discussed, such as current flow and space-charge phenomena for cases in which electrons execute cyclic orbits. Extensive experimental data are included.