CONDUCTION THROUGH GASES 157 



that these ions vary In size, some probably being 

 molecules of the gas, and others molecules of the 

 metal or even dust disintegrated from its surface. 

 As the platinum is still further heated, negative 

 ions also come off, ultimately in large excess. In 

 vacuo the negative leak from platinum and carbon 

 filaments is very large — from carbon it may even 

 amount to as much as an ampere of current from 

 each square centimetre of surface. The negative 

 ions are then of sub-atomic dimensions, and are 

 identical with the electrons otherwise obtained. 

 H. A. Wilson has shown that, at the lower tem- 

 peratures at which the negative leak occurs, it is 

 very largely due to the effect of hydrogen absorbed 

 in the platinum, and liberated under the action of 

 the heat. At the highest temperatures, however, 

 the electrons due to the wire itself seem to be 

 much more numerous than those depending on 

 the presence of hydrogen, and to the metal itself 

 we must then look for their source. 



The emission of electrons at high temperatures 

 is not confined to solids. Thomson finds that 

 sodium vapour also gives off a large supply, and 

 the effect seems to be common to all kinds of 

 matter at a white heat. Carbon is particularly 

 efficacious, perhaps because it can be raised to a 

 higher temperature than can most metals. It is 

 easy to demonstrate the existence of a measur- 

 able current from one limb of the carbon filament 

 of an ordinary incandescent electric lamp to an 

 insulated plate placed between the limbs. 



Owing to the emission of electrons by an 

 incandescent wire or carbon filament along which 

 a current flows, the effective current-carrying area 

 of the wire is increased. In vacuo a considerable 



