﻿Method of Investigating Thermionic Emission, 367 



they were found to be attracted by a positively electrified but 

 not by a negatively electrified rod, while at temperatures 

 near the melting-point of platinum neither rod attracts the 

 filament. It is of course well known that platinum in air 

 emits chiefly positive ions at low temperatures and an excess 

 of negative ions at high temperatures. 



Experiments with Metal Filaments. 



The knowledge that a glowing filament is attracted by a 

 rod charged with electricity of the same sign as the ions 

 emitted by the filament, and that the removal of a rod charged 

 with electricity of opposite sign causes a movement of the 

 filament provided there is a solid dielectric between them on 

 which to receive the ions, has been used by the author as the 

 basis of a qualitative method of investigating thermionic 

 emission. 



As the results obtained from experiments on the alloy 

 nichrome are typical of those obtained from the metals and 

 alloys mentioned below, a detailed account for this case only 

 will be given. 



A long filament (40 to 50 cm.) of No. 40 S.W.G. nichrome 

 wire was attached by silver solder to the leading-in wires 

 of a metal-filament lamp from which the glass envelope and 

 original filament had been removed (see fig. 3). 



This method of suspension was found to be most convenient 

 -p.^ o and possesses the advantage that it is readily 



_ * connected to the ordinary lighting circuit by 



P means of the usual bayonet-type holders. 



X \ At a dull red-heat the filament was readily 



i \ attracted by a rod charged with electricity of 



4"-^ either sign, showing that there was no appreci- 



able thermionic emission of either sign at this 

 I temperature. 



\\ At a bright red-heat (current 1*5 amps.) the 



U filament was attracted by a positively charged 



rod, but not by a negatively charged one. A 

 sheet of glass was then interposed between the 

 filament and the charged rods. This had no influence on 

 the action of the positively electrified rod, but the filament 

 was attracted towards the glass screen when a negative rod 

 was removed from its neighbourhood. 



For the purpose of investigating the emission in air at 

 pressures less than the atmospheric pressure, and in other 



