216 Mr. E, V. Appleton and Dr. B. van der Pol on 



Plate X. b. Immediately after the circuit is closed the filament 

 has still the temperature corresponding to the equilibrium 

 state where no electrons are drawn away, and at first the 

 emission current is large. The emission, however, imme- 

 diately afterwards causes the temperature to fall, which in 

 its turn causes a decrease in emission current. Plate X. a is 

 a check oscillogram in which an ordinary resistance was sub- 

 stituted for the diode. 



It has been shown above that in a triode of the ordinary 

 type marked variations in filament temperature and elec- 

 tronic emission take place when the anode current varies 

 slowly. Some further points in connexion with this effect 

 when static currents and voltages are employed will now be 

 considered. 



Fis:. 10. 



■|i|i|iMi— |i|i|in 



If we examine the case of a hypothetical filament with 

 two centres of emission as indicated in fig. 10, from which the 

 total thermionic current (*i + i 2 ) is collected, it may be shown 

 that the values of current flowing in the two sections of the 

 filament circuit are 



3 + a ,. 



M + h) 



2o + 



4(l + a) 

 l+3a 



(h + h), • 



(11) 



4(1 + «] 



where i is the filament current before the emission is col- 

 lected and u=-) is the ratio of the emission from the nega- 



tive and positive halves of the filament respectively. If we 

 take the experimentally determined values for i + and i_, that 

 is to say i — 0'^{i 1 -\-i 2 ) and z + 0'7 T?'i -h2 2 ), we find that a = 9, 

 which would suggest that the emission from the negative 

 half of the filament is nine times the emission from the 

 positive half. Such a value, of course, represents an 

 imaginary case, and it cannot be expected that the dif- 

 ference in emission between the two halves of the filament 



