Oscillations in the current are reported in a thermionic triode with an auxiliary low-pressure, inert-gas discharge. This oscillation is not accompanied by an oscillation in the circuit of the auxiliary discharge. To interpret the results, it is assumed that the ion motion is due to diffusion, while the motion of the slow electrons is rectilinear. Under these assumptions a neutral plasma can only exist in the gap if electrons are captured in a potential well. The oscillations appear when the potential well (near the collector) which retards electrons from the plasma disappears.