Disintegration of the Aluminium Cathode. 347 



potential of distilled zinc and bismuth ? It is not possible at 

 this stage to offer any explanation of the precise way in 

 which absorption of gases by the surface affects the contact 

 potential, and indeed whether it is due to mere absorption or 

 to something of a more definite chemical nature. 



Summary* 



An investigation has been made on the contact difference 

 of potential between zinc and bismuth, both distilled in 

 vacuo, and platinum. Initially, there is but little contact 

 difference of potential between the distilled metal and 

 platinum. The admission of a trace of air into the vacuum 

 causes a great increase in the contact difference of potential. 

 The change is located at the surface of the distilled metal, 

 which becomes more electropositive. A maximum is reached 

 after which the contact potential difference decreases to a 

 steady value which is scarcely affected by the presence or 

 absence of air. 



A connexion between these results and some recent work 

 on the photo-electric effect has been pointed out. 



The Rice Institute, Houston, Texas. 

 May 30th, 1914. 



XLI. Disintegration of the Aluminium Cathode. 

 By L. L. Oampbkll *. 



IN 1891 Crookes f made the first quantitative determi- 

 nation of the rate of disintegration, or " electrical 

 evaporation, " as he termed it, of a number of metals used 

 as cathodes in discharge-tubes containing rarefied air. He 

 found that the following series represented the relative rate 

 of disintegration of the cathode, in descending order : Pd, 

 Au, Ag, Pb, Sn, Pt, Cu, Cd, Ni, Ir, Fe. The disintegration 

 of the Al and Mg cathodes he found to be zero. This lack 

 of disintegration, or " spluttering," on the part of the 

 aluminium cathode has made its use in spectrum and other 

 discharge-tubes very general. 



Shortly after the discovery of the noble, or monatomic 

 gases, argon, helium, neon, xenon, krypton, it was found by 

 Orookes J, Travers §, Baly ||, Soddy and McKenzie II, 

 Claude **, and others, that the aluminium cathode 



* Communicated bv Sir J. J. Thomson, CM., F.R.S. 



t Proc. Roy. Soc. 1. p. 88 (1891). 



% Zeitschr.f.2>hys. Chemic, xvi p. 370 (1895). 



§ Proc. Roy. Soc. lx. p. 449 (1897). 



|| Ibid, lxxii. p. 84 (1903). 



«[ Ibid. lxxx. Ser. A, p. 9l> (1908). 



** Compt. Rend, cliii. p. 713 (1911). 



