﻿-366 Mr. F. Lloyd Hop wood on a Qualitative 



of the beta rays from 20 milligrammes of pure radium 

 bromide, and to both hard and soft X-rays from a powerful 

 X-ray tube, it was found that, when due precautions were 

 taken to prevent the discharge of the charged rods by the 

 atmospheric ionization produced by the radiations, any 

 ionization of the residual gas in the lamp had no appreciable 

 influence on the displacement of the filament. 



Galvanometric measurements of the thermionic current 

 between one filament when raised to incandescence and the 

 other (cold) filament, when the cold filament was maintained 

 first at a positive potential and then at a negative potential, 

 showed a comparatively large negative emission, but gave no 

 certain indication of a positive emission from the hot filament. 

 Air was then readmitted into the lamp and pumped out 

 again by means of a Topler pump. It was noticed that 

 when the filaments were first raised to a dull red heat after 

 the re-evacuation of the lamp, a positively charged rod 

 produced a divergence on approach and a negatively charged 

 rod a displacement of the loops on removal, thus showing 

 that at the low temperature at which this took place, the 

 filament emitted positive ions only. 



This effect was only temporary and disappeared on con- 

 tinued heating. 



The experiments of Richardson, Wilson, and others* 

 show, however, that in similar cases a small permanent 

 leak of positive ions still persists after the larger temporary 

 one has disappeared, and that this leak (which can only 

 be measured by an electrometer method) is very much 

 greater at high than at low temperatures, although at the 

 higher temperatures it is much smaller than the negative 

 emission. 



Now in the present experiments the inducing charges, 

 though at high potential, are very small f , so we may conclude 

 for the reasons given above that the null effect at high 

 temperatures is due to the emission of sufficient positive and 

 negative ions to remove the induced charges, while the 

 positive emission at lower temperatures is too small to 

 dissipate the charges induced by the negative rod sufficiently 

 quickly to prevent the motion of the filaments. 



Experiments made with glowing platinum filaments in the 

 open air tend to confirm this conclusion, for at a red heat 



* Vide J. J. Thomson, ' Conduction of Electricity through Gases/ 

 2nd. ed. p. 214; and Schottky, Thys. Zeitschr. July 1914. 

 \- t Glass rods rubbed with silk, and ebonite rods rubbed with flannel, 

 were used throughout. 



