ivlien an Electron escapes from Surface of a Hot Body. 217 



obtained ^vitli tungsten and osmium. The pressure in the 

 tube during the carbon experiments varied from 1 x 10" 4 mm. 

 with the wire hot to 3 x 10~ 5 mm. with the wire cold, ap- 

 proximately. This behaviour o£ carbon verifies for the value 

 of c£, found when inert gases are known to be present, an 

 observation that Langirmir made on tungsten. He found 

 that the presence of argon in the experimental tube did not 

 affect the value of b. Argon is inert to tungsten. It would 

 be interesting to find out if any pressure of inert gets w T ould 

 affect the value of c/>. This v T us not attempted in the present 

 investigation. 



3. Nature of Surface Films. 



It is probable that active gases r.fTect the value of $ 

 through the formation of surface films. It is interesting 

 and important to consider the nature of these films. The 

 work of Richardson and Cooke shows that the heating effect 

 measured under conditions that are not ideal is greater 

 than the cooling effect measured under ideal conditions. 

 It is probable that the heating effect, like the cooling effect, 

 is increased in the presence of active residual gases. If 

 the film acts merely as a high resistance through which 

 the escaping electron must go, then the work that an 

 electron does in escaping would be exactly represented by 

 an equivalent Joule heating effect. Since we may regard 

 the film as at all times in thermal equilibrium with the hot 

 surface, this Joule heating would act to decrease the cooling- 

 due to the escape of electrons, and we should expect the 

 cooling effect to be decreased by the film. This is, however, 

 contrary to observation. The alternative view is to regard 

 the film as charged. A charged film might be produced by 

 the ionization of surface molecules of a surface compound 

 and the loss of electrons to the interior of the metal, or the 

 arrangement of ions into a double layer with positive ions 

 on the outside. The field within such a film would oppose 

 the passage of escaping electrons and accelerate the passage 

 of entering electrons. Since both the heating and cooling 

 effects are increased, there must be some such field acting at 

 each surface. There is reason to expect such an arrange- 

 ment of double layers at a metallic surface. The same force 

 which retains electrons within a metallic surface w y ould also 

 tend to retain negative ions within that indefinite region 

 called the "surface." Positive ions would gravitate to 

 the outer portion of the region occupied by the ions and 

 molecules that make up the so-called film. 



