the Form of Free Triode Vibrations. 219 



no series resistance. Thus in the case of a long or very thin 

 filament where E is large, the energy supplied to the fila- 

 ment may be substantially increased when current is taken 

 from the tube, resulting in further increased emission and 

 so on. There thus arises the possibility of a new type of 

 self-sustained discharge in which the cathode is maintained 

 at a high temperature purely by the heating effect of the 

 emission current itself. 



An arrangement similar to that of fig. 11 would be 

 necessary to start such a discharge. 



Summary. 



(a) The temporal variations of the anode current in a self- 

 excited triode generator are studied oscillographically and the 

 results interpreted in terms of an oscillation characteristic 

 obtainable from the ordinary triode characteristics. A 

 method of estimating the amplitude of the anode current 

 variations in terms of the oscillation characteristic and the 

 •circuit constants is described. Oscillograms illustrating 

 cases of departure from the simple theory are explained by 

 taking into account the particularly large conductance of the 

 space between grid and filament during one part of the grid 

 voltage cycle. 



(b) On closing the anode circuit of a typical triode the 

 mean -temperature is reduced because the loss of heat due to 

 electronic evaporation is not counterbalanced by the extra 

 heat supplied by the anode c irrent in passing through the 

 filament. A reduction in emission results from the tall of 

 temperature, and because of the finite thermal capacity of the 

 filament an appreciable time is required for the emission to 

 fall to its equilibrium value. Such changes in emission take 

 place whenever the anode current is varied slowly. 



(c) The effect of the temperature variations mentioned in 

 (/>) is considered in a case in which the anode current varies 

 periodically as in a self-excited generator, and it is shown 

 that with a filam uit of ordinary size the effect will only be 

 marked at extremely low frequencies, e.g. 16 per second. 

 Oscillograms taken at this frequency show the cooling of the 

 filament and consequent reduction in emission during the 

 saturation phase of the anode current. With an extremely 

 thin filament the effect would be p onounced even at tele- 

 phonic frequencies. For very high frequencies the emission 

 will be sensibly constant and thus the dynamic characteristic 

 will in general be slightly different from Hi* one ordinarily 

 deduced from a knowledge of the static characteristic and 

 the circuit constants. 



Q-2 



