﻿364 Mr. F. Lloyd Hopwood on a Qualitative 



equilibrium state is attained. On bringing up a negatively 

 charged rod, no further electronic emission can take place 

 and the loops diverge in a similar manner and for the same 

 cause as the leaves of an electroscope. When a 'positively 

 charged rod is brought up, the conditions are made more 

 favourable to the electron emission, and the negative charge 

 induced on the filament by the rod escapes from the filament 

 to the walls of the lamp, giving a negative charge to the 

 inner surface of the bulb on the side nearest the rod. Thus, 

 owing to the escape of the induced charge, the loops are 

 unaffected by the approach of a positively electrified rod. 

 Before the removal of the rod, the filament will be in 

 equilibrium under the joint action of the charges on the rod 

 and on the inner surface of the bulb. On the removal of the 

 positively charged rod, the negative charge on the walls 

 induces a positive charge on the filament and produces the 

 observed displacement of the loops. 



Touching the bulb with the hand or playing a Bunsen 

 flame on it, neutralizes the negative charge on the inner 

 surface by permitting the accumulation of a positive charge 

 on the outer surface of the bulb. The charging of the 

 electroscope is obviously due to the same cause. 



The response to the motion of the hand will be due partly 

 to a redistribution of the charge on the walls and partly to a 

 change in the potential difference between filament and 

 walls. 



Null Effect at High Temperatures *• 



When various carbon filaments were heated above their 

 ordinary temperatures by running the lamps above their 

 normal voltage, they were found to be insensitive to the 

 motion of charged bodies in their neighbourhood, although 

 they were very sensitive to such motions at lower tem- 

 peratures. 



It w T as sometimes found that filaments of high candle- 

 power lamps were also insensitive w T hen run at their normal 

 voltage, but sensitive at lower voltages (temperatures). 



It was thought that the null effect in these cases might be 

 due to one of the following causes : — 



* In a preliminary note on this effect which appeared in ' The 

 Electrician ' of July '61, 1914, a different explanation from that given in 

 the present paper was advanced, but in the light of further experiments 

 has had to be abandoned. 



