the Form of Free Triode Vibrations. 21& 



Using the values given above, we find for the fall of 

 temperature 



f=7 degrees centigrade, 



in which case the emission would be considerably reduced. 



In view, however, of the many unknown functions still 

 occurring in the formulae and the fact that tungsten does 

 not follow the Stefan-Boltzmann law accurately*, these nu- 

 merical results are to be considered as only very approxi- 

 mate. Nevertheless the theory set out above, incomplete as 

 it is, throws some light on the different causes which tend to 

 increase or decrease the mean filament temperature. 



The above considerations show that in the case of filaments 

 of ordinary size the thermal changes will only be appreciable 

 for extremely low frequencies, e. g. 16 per second. An 

 oscillographic study was made of a circuit of such a fre- 

 quency with the aid of a string galvanometer and the cooling- 

 effect was found to be clearly marked. As the oscillograms 

 at the same time yielded other points of interest in connexion 

 with the effect of the mutual inductance on the form of the 

 anode current variations, these results will now be described 

 in detail. 



Fig. 9. 



-|i|i|i|i|-1-M»- 

 M 



A circuit with the connexions of fig. 9 was used. L x is a 

 multilayer coil (axial length 9*1 cm., inner diameter 9*1 em.. 

 outside diameter 18*2 cm., total number of turns 1500). 

 L 2 is a similar coil which could be made to slide inside the 



* Langmuir, Phys. Rev. vol. xxxiv. p. 401 (1912). 



