The transient behaviour of thermionic filaments with temperature-limited emission

Abstract

The anode current of a saturated diode is very sensitive to changes in filament current. A diode may thus be incorporated in an incremental ammeter or used in servomechanisms as an element responsive to current changes of small amplitude. For the latter purpose the time response of the diode is important. The factors determining the time response for small changes in filament voltage or current are investigated and procedures are described for calculating and for measuring its value. It is shown that the time-constant of a filament for temperature changes not exceeding 100° C depends on whether power is supplied to the filament at constant voltage or at constant current, being somewhat greater for the former than the latter. It is also shown that the time constant is independent of the filament length, directly proportional to the diameter and dependent on the temperature. It depends on the filament material only in so far as this affects its specific heat. For tungsten (and thoriated tungsten) filaments the value of the time constant may lie between about 0.03 to 0.3 sec.