Kinetic Energy of Positive Ions emitted by Hot Platinum. 355 



size of vapour in the liquid, we see that we may expect in the 

 immediate neighbourhood of the critical temperature in the 

 proof-tube for critical researches, formation of mist and rain 

 in the vapour and small vapour bubbles in the liquid. As 

 every one knows, these phenomena are observed. Teichner for 

 instance observes : " It is remarkable that this rain breaks 

 out in both phases : in the vapour phase small drops of fluid 

 fall down, and in the liquid small bubbles of gas ascend " *. 



XZXII. The Kinetic Energy of the Positive Ions emitted by 

 Hot Platinum. By F. 0. Brown, Experimental Science 

 Felloiv, Princeton University f. 



T 



HE object of the present paper is to set forth an investi- 

 gation into the magnitude and the mode of distribution 

 of the kinetic energy of the positive ions emitted by hot 

 platinum. 



The method of investigation was the same as that used by 

 Richardson and Brown J for the negative ions. It consists 

 in measuring the rate of charging up of a plate opposite a 

 parallel plate containing a strip of the hot metal, both plates 

 being initially at zero potential. The ions from the hot 

 metal cause the opposite plate to acquire a positive potential 

 tending to stop further charging up. From the rate at 

 which the current varies with the potential, both the mean 

 value of the kinetic energy and the way in which it is dis- 

 tributed among the different particles can be determined. 

 For a detailed description of both the theory and the practice 

 of the method, reference must be made to the previous paper. 

 As is there pointed out, the method only yields information 

 abont that portion of the kinetic energy of the ions which 

 depends on their velocity components normal to the emitting 

 surface. 



It was shown (he. cit.) that whatever the law of distribu- 

 tion of energy may be, the current between the plates 

 (supposed infinite) will be given by 



i = c~j~=ne\ F{u )du \ /{v^dvA f(w )dw , (1) 



( ' «- /o'l- »/ -00 J-OO 



v hv 

 where c is the capacity of the upper plate and its connexions, 



* G. Teichner, Ann. d. P/iys. xiii. p. f>97 (1904). Compare also: 

 IT. Kamerlingh Onnes and G. EL Fabius, Repetition of de Heen's and 

 Teichner's experiments on the critical state : Communications from the 

 Physical Laboratory at the University of Leiden, Nr. 98, 1907. 



t Communicated bv Professor Richardson. 



+ Phil. Mag. Sept. '1908, pp. 355-366. 



