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Under the assumption of absolute vacuum in a tetrode, the space-charge depression in the screen-grid anode region will at sufficiently high current prevent secondary electrons released at the anode from reaching the grid, thus making the anode-current, anode-voltage characteristic similar to that of a pentode. Under pressure conditions present in ordinary high-vacuum tubes, positive ions formed by collisions between electrons and residual gas molecules will be trapped in the potential trough between the screen grid and the anode. Provided there exists no transverse ion-sweeping fields from beam-forming plates (as in the beam-power tetrode) or from charged end insulators, ion trapping proceeds automatically until the electron charge is neutralized. The effects of ion-trapping are discussed and a pulse method is described for measuring the conditions with and without presence of ions and for studying the ion-buildup process. Plasma ion oscillations have been observed.