Maintenance of Electron Emission from the Filament of a Triode after Its Low Tension Supply Is Disconnected

It is found that when abnormally high anode or grid voltages are applied to certain types of dull emitter receiving triodes so that the anode or grid current is a considerable fraction (greater than fifteen per cent) of the normal filament current, the electrode currents persist even after the low tension cell feeding the filament is switched off. But this electron emission from the filament without the low tension battery is maintained at normal and even subnormal electrode potentials if the tube is made to generate oscillations under high efficiency régime. It is shown that the maintenance is caused by the heating of the filament by the anode or the grid current, as the case may be, passing through it. A high negative grid current of the order of a few milliamperes flows during a small fraction of each cycle when the tube is generating such oscillations, and this is shown to be the result of secondary electron emission from the grid.