﻿Thermionic Currents from Tungsten. 821 



Discussion of Results. 



Figure 5 may also be nsed to illustrate the typical results 

 of all the experiments. When the observations are plotted 

 it is found that they fall into four groups, represented by 

 curves L, II., III., IV., as follows : — 



I. The first currents in four lamps (3, 4, 5, 7) (cf. also 

 fig. 4). The first currents in the other lamps were 

 not measured. 

 II. (a) The last series with lamp 1 (cf. fig. 2). 



(b) The currents after the liquid air was removed 



from lamp 6. 



(c) The largest currents in lamp 5 (cf. fig. 4). 



(d) The first currents after lamp 7 was sealed. 



III. The permanent emission in lamp 7, which was not 

 heated much above 2500° K. This would also 

 include the currents from filament "B," lamp 6, 

 before the liquid air was removed. 

 IV. The currents observed after heating the filaments to 

 a very high temperature, 2700° K. or more, under 

 the best vacuum conditions (lamps 1, 2, 6 "A"). 



These variations in the thermionic emission seem to be the 

 results of progressive changes in the surface conditions *. 

 When the filament is first heated its surface is probably 

 slightly oxidized, and otherwise contaminated by impurities. 

 No special precautions to clean the filament were taken 

 before it was sealed in the lamp. After a good vacuum has 

 been secured, heating the filament for a sufficient time to a 

 temperature above 2700° K. removes the impurities, and 

 then the emission is represented by IV. This condition is 

 permanent thereafter provided the vacuum is maintained, 

 but under the following circumstances the emission changes 

 to II. , which is not much larger than the original I. : 

 (a) if gas is liberated inside the lamp by excessive heating 

 and bombardment of the anode (lamp 1), or (b) if the fila- 

 ment is oxidized by allowing small amounts of water vapour 

 to enter the lamp, as in 6. 



The removal of impurities from the surface seems to be 

 indicated quite clearly by the observations made when the 

 filament in lamp 7 was first heated above 2000° K. after 

 sealing. This changed the emission permanently from II. 

 to III. Since the lamp had been sealed under good vacuum 

 conditions there was no fresh supply of gas by which the 



* Cf. Lang'inuir, loc. tit. 



