The presence of the positive ions affects the action of the tube in two ways. In the first place the ions are of opposite electrical sign to the electron space-charge, so that the effect of the latter in dis couraging the escape of electrons from the region near the filament is much reduced. The presence of gas, therefore, facili tates the flow of current through the tube at lower anode poten tials. The positive ions, in fact, play the same part as the positively charged inner grid in a tetrode of the first type de scribed above. In the second place the presence of positive ions causes current to flow to the grid when the latter is charged negatively with respect to the filament. Since, however, with in creasing negative potentials the anode current, and thus the rate at which the positive ions are produced, is decreased, the positive ion current to the grid is similarly reduced. This is illus
trated by typical grid and anode current characteristics for a soft tube in fig. 15. For conditions represented by point A the grid filament space acts like a negative resistance, in that, with de creasing applied potential, increasing current flows, so that if an oscillatory circuit is connected across it, phenomena similar to those experienced in connection with the use of with a dynatron may be reproduced.
Indirectly-heated-cathode Valves.—In order to dispense with the use of accumulators for filament heating, valves with thick filaments, heated from the A.C. supply mains, may be used. Such a practice, although successful for the amplifying stages of a receiver, is quite unsuitable for the detector stage, for which special valves have to be used if the A.C. heating is to be retained throughout. These special valves have, as cathode, a nickel cylinder coated with barium or strontium oxide which is heated by the thermal radiation from a hot tungsten spiral filament inside the cylinder to a temperature sufficiently high to cause thermionic emission. Such valves are usually designated as indirectly-heated-cathode tubes. Apart from the highly special ized structure of cathode the electrodes conform very closely to normal design.