1915] on Electrons and Heat 449 



the anxiliarj field used to drive them from the hot wire to the strip. 

 This, however, is easily determined and allowed for. 



I have now indicated to jolt three independent methods of 

 deducing the values of the latent heat of emission of the electrons. 

 Let us see how the latest and most accurate values obtained by these 

 methods agree with one another. The numbers found, and the 

 names of the experimenters responsible for them, are shown in the 

 following table : — 



Values of Latent Heat of Emission reduced to Equivalent 

 Temperatures. 



1. From the temperature variation of the rate of emission. 



Tungsten (Langmuir) 10-5 x 10^ - 11*1 x 10^ calories 



per Mol. 

 Tungsten (K. K. Smith) 10-94 x 10"^ calories per Mol. 

 Platinum (various) 12x10^-16x10"^ calories per Mol. 



2. From cooling due to emission. 



Tungsten (Cooke and Richardson) 11*24 x 10^ calories 



per Mol. 

 Tun-gsten (Lester) 11*04 x 10* calories per Mol. 

 Platinum (Wehnelt and Liebreich) 13 * 9 x 10* - 14 • 5 x 10 



calories per Mol. 



3. From heating due to condensation. 



Platinum (Richardson and Cooke) 13*5 x 10* calories 

 per Mol. 



Unfortunately, the vacuum value for platinum given by the first 

 method is still uncertain, owing to complications caused by gaseous 

 contaminants. Except for this, the agreement between the different 

 methods leaves nothing to be desired. 



We come now to the very interesting question of the velocity 

 and kinetic energy which these electrons possess when they are 

 emitted. The fact that they are electrically charged enables us to 

 find out a great deal more about their emission velocities than we 

 can do in the corresponding case of the emission of ordinary mole- 

 cules. By applying an external electric field we can influence the 

 motion of the emitted electrons, and the precise nature of the effect 

 exerted by the field depends on the velocity with which the electrons 

 are shot off from the hot body. It is clear that we have no such 

 method of controlling the motion of ordinary molecules. 



I shall now consider one of the arrangements which has been 

 used in applying these principles to the analysis of the emission 

 velocities. The hot emitting surface is a small strip of platinum, 

 electrically heated, which lies at the centre of a much larger metal 

 plate. The upper surfaces of the strip and the plate are flush with 



