﻿368 Qualitative Method of Investigating Thermionic Emission, 



gases, the filament was mounted in the apparatus shown in 



fig. 4. The use o£ the ground-glass stopper facilitated the 



replacement of one filament by another, while 



the side tubes permitted the introduction of 



different gases and the regulation of the 



pressure by attachment to a pump. The hot 



nichrome showed the attraction produced by 



a positively charged rod and the kick due to 



the removal of a negatively charged rod in 



air at atmospheric pressure and at pressures 



down to less than one centimetre of mercury. 



It gave similar indications in hydrogen 

 and C0 2 . 



We conclude that nichrome at a bright red- 

 heat in air, hydrogen, and C0 2 emits a large 

 number of positive ions, and that its negative 

 emission is insignificant. ^ V~~ 



Of the following metals and alloys all were 

 tested in air and a few in hydrogen and C0 2 . In every case 

 they gave a positive emission. 



The substances examined were : — 



Iron, nickel, copper, nichrome (an alloy of nickel and 

 chromium), brass, phosphor-bronze, silicon-bronze, platinoid,, 

 eureka, tinned copper, and " galvanized '' iron. 



So far as the writer is aware, the emissions from most of 

 the alloys mentioned above have not previously been in- 

 vestigated. 



Advantages of the Method. 



The method of investigation outlined above is very suitable 

 for a rapid qualitative examination of the emissions from a 

 large number of substances in air, and other gases, at various 

 pressures. It requires no special manipulative skill, and use 

 is made of only such apparatus as is generally found in a 

 Physics Laboratory. It provides some striking lecture 

 experiments. Incidentally, it was noticed that incandescent 

 filaments mounted as above (fig. 3) are very susceptible to 

 the approach of a magnet, and the laws governing the 

 mechanical action of a magnet on current-bearing conductors 

 may be readily demonstrated to large audiences by their 

 use. 



Summary* 



Experiments are described illustrating the difference in 

 behaviour of incandescent carbon filaments under the influence 

 ©f small positive and negative charges. Explanations are 



