﻿Method of Investigating Thermionic Emission. 365 



a. An anomalous increase in the nexural rigidity of the 



filament owing to some change in structure due to 

 a molecular rearrangement such as is known to 

 occur in iron and silica at high temperatures ; 



b. The residual gas in the lamps becoming ionized to 



such an extent by the hot filaments that it acts as 

 a protective conducting-sheath towards external 

 electrostatic fields ; 



c. The emission of both positive and negative ions in 



sufficient quantities to neutralize an induced charge 

 of either sign on the filament. 



Experiments were carried out to test these hypotheses. 



Some difficulty was experienced in devising an experiment 

 to test the mechanical properties of a carbon filament at high 

 temperatures, but finally the lamp shown in fig. 1 was used 

 for this purpose. The filament was subjected to stress by 



Fig. 1. 



Q 



c 



V 



Fig. 2. 



oooo 



placing it in a fairly strong magnetic field while the heating- 

 current passed through it. 



No evidence of any increase of rigidity with rise of 

 temperature was obtained, the yield of the filament in- 

 creasing with rise of temperature for all the temperatures 

 used. 



To test hypotheses (b) and (c), an incandescent lamp with 

 v1 vo similar unanchored "hairpin" filaments was used (see 

 fig. 2). On subjecting the residual gas to the ionizing action 



