﻿S22 Negative Thermionic Currents from Tungsten. 



filament could again be contaminated. It should be recalled 

 that no emission greater than II. could be obtained in lamp 5, 

 and it seems quite likely that continued heating was futile 

 in this particular case, because the impurities were being 

 renewed as rapidly as they were driven off. 



Barring lamp 3, which was burnt out at the end of the 

 first series, lamps 5 and 7 were the only ones in which the 

 currents represented by I V. were not obtained, and it seems 

 significant that these were the lamps which were not heated 

 much above 2500° K. Moreover, as already shown in 

 Table II., one filament in lamp 6 gave the large currents IV., 

 but immediately afterwards, and under the same vacuum 

 conditions, the other gave currents approximately the same 

 as J II. The former had been heated to 2700° K., whereas 

 the latter had not been heated above 2500° K. This is what 

 might be expected if the filaments contained an impurity 

 which could not be removed except at very high tempera- 

 tures. If the temperature were not raised too high the 

 impure surface would give continuously currents such as III., 

 but after the impurity had been driven out, the permanent 

 thermionic emission, IV., would be characteristic of the pare 

 metal. 



One of the most interesting results of these experiments 

 is the enormous range of validity established for the emission 

 formula 



i = AlVf , 



when the conditions are such that no change in the character 

 of the emitting surface is believed to occur. The formula is 

 shown to hold good whilst the current is varied by a factor 

 of 10 12 , the corresponding range of temperature being from 

 1050° K. to 2300° K. An equally good agreement is shown 

 with the formula i = GT 2 e~ d/T , with C and d constants and 

 d equal to 52000 ; so that it is not possible to distinguish 

 between the relative merits of these two formulae with the 

 data so far obtained. 



This investigation was begun under the direction of 

 Professor Richardson, and I wish to express my appreciation 

 of his assistance and advice during its progress. My thanks 

 are also due to Dr. Irving Langmuir for supplying the wire 

 used, and for furnishing the results of special experiments 

 with it. To Dean W. F. Magie I desire to acknowledge my 

 indebtedness for helpful criticism. 



Palmer Physical Laboratory, 

 Princeton, N.J. 



