Maintaining a Constant Reading on an Ammeter in the Filament Battery Circuit of a Thermionic Triode

The distribution of current along the filament of a thermionic valve is not constant on account of the emission which takes place from the filament to the anode. When the valve is not emitting, ammeters in each filament lead read the same value, but when emitting they are necessarily different, and for the usual methods of connection between the anode battery and filament they are also both different from the non-emitting reading. For accurate tests it is necessary to have a constant reading for comparative tests, and as the filament current cannot be maintained constant, a circuit has been devised in which there is no change on either ammeter reading when the anode battery is switched on. This is the filament battery current, which, when constant for the cases of non-emitting and emitting, provides a standard for comparison between different tests.