198 Mr. H. H. Lester : Determination of Work Function 



should involve a loss of thermal energy, due to the fact that 

 energy is rendered latent by the potential difference at the 

 surface, and that this loss ought to increase rapidly with 

 the temperature. Wehnelt and Jentzsch * announced the 

 discovery of such a cooling effect, and attempted its mea- 

 surement. However, they seem to have neglected to 

 compensate for the disturbing action of the thermionic 

 current in their bridge system, so that the effect they 

 measured was considerably obscured by the thermionic 

 current disturbances. Cooke and Richardson f found such 

 an effect in 1913. They described a method of measure- 

 ment, and published values on the cooling effect, for osmium 

 and tungsten. Before this, however, viz. in 1910, the same 

 experimenters had announced the discovery of a converse 

 heating effect J and had published values for a series of 

 metals. The value of <£ was calculated from both the 

 heating and the cooling effects, and was found to be of the 

 expected order of magnitude. 



The present paper contains a discussion of their work 

 on the heating effect, and presents an extension of their 

 measurements of the cooling effect. Some related features 

 are discussed. The paper is divided as follows : — 



I. Experiments of Richardson and Cooke. 



II. Measurement of the cooling effect for carbon, molyb- 

 denum, tantalum, and tungsten. 



III. Related features : 



1. Identification of b with <£. 



2. Relation of <£ to contact potentials. 



3. Effect of gases on <£. 



4. Nature of surface films. 



5. A new method for determining the temperatures 



of hot filaments. 



I. Experiments of Richardson and Cooke. 



These experiments carried out in this laboratory showed 

 the existence of the heating effect. It is not claimed for 

 them that they show more than the order of magnitude. In 

 fact, Richardson was inclined to the view that, owing to the 

 way in which the contact potential entered, the effect found 

 rather measured the cooling effect at the surface of the hot 



* Ann. der Physik, (3) vol. xxviii. p. 537 (1909). 

 t Phil. Mag. April and Sept. 191 3. 

 + Phil. Ma?. July 1910 and April 1911. 



