﻿Method of Investigating Thermionic Emission. 363 



leaves of an electroscope, the divergence disappearing on the 

 removal of the charged rod. 



If a negatively charged rod is brought near the lamp when 

 the filament is glowing, the loops diverge, the divergence 

 again disappearing on the removal of the rod, or if the 

 filament touches the glass walls of the lamp. 



When, however, a positively charged rod is brought towards- 

 the glowing filament, no movement of the loops occurs. On 

 rapidly removing the positively charged rod from the neigh- 

 bourhood of the glowing filament the loops diverge. If the 

 divergence is sufficiently great for the loops to touch the 

 glass walls of the lamp, they immediately spring back to 

 their original position, but if they do not touch the walls, 

 they will remain in the displaced position, in some cases for 

 several minutes. When in the displaced position produced 

 in this way, the loops respond very readily to the movement 

 of any bodies whether charged or uncharged, in their 

 neighbourhood — "twiddling'' the fingers near the lamp sets 

 them into violent vibration. Placing the bulb momentarily 

 in contact with the metal cap of an electroscope gives a 

 negative charge to the leaves. 



Should the bulb be touched by the hand or a Bunsen flame 

 be rapidly passed over it, the displacement of the loops, their 

 response to the movement of an uncharged body, and the 

 ability of the bulb to charge an electroscope on contact, all 

 disappear. 



The above effects may be explained as follows : — 

 It is well known that when a carbon filament is raised to 

 incandescence in the neighbourhood of a cold conductor 

 maintained at a positive potential, both being enclosed in an 

 evacuated vessel, a continuous stream of electrons passes 

 from the hot filament to the cold conductor. The emission 

 under these circumstances has been studied in great detail 

 by Richardson* and others. If the cold conductor is not 

 maintained at a positive potential by external means, the 

 emission from the hot filament proceeds until a certain 

 limiting difference of potential between the filament and cold 

 conductor is attained. This limiting potential depends on 

 the geometrical configurations of the cold conductor and the 

 hot filament, and its value has been obtained in certain 

 cases, both theoretically and practically, by W. Sehottky f. 

 In the case of an ordinary carbon-filament lamp, this limiting 

 potential difference is quickly established between filament 

 and walls of bulb when the filament is glowing, and an 



* llicliardson, Phil. Trans. 1903. 



+ Deutsch. Phjs. Gessell., Verh. xvi. 10, May 1914. 



