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LXXIV. The Discharge of Electricity through Gases and the 

 Temperature of the Electrodes. By J. A. Cunningham. 

 B.A. {R.U.L& Camb.), A.R.C.Sc.L, 1851 Exhibition 

 Scholar, Cavendibh Laboratory, Cambridge *. 



Introduction . 



THE conduction of electricity through hot gases and 

 vapours has been the subject of investigations by 

 Becquerel, Blondlot, Grove, Maxwell, and Hittorf. In 1890 

 Prof. Thomson t reinvestigated the whole subject, and showed 

 that the heating of the electrodes was an essential part of the 

 phenomenon. Recently Dr. H. A. Wilson J has investigated 

 the conductivity of air and salt vapours up to about 1300°. 

 His electrodes were, of course, also hot, and from his previous 

 experiments on flames he had concluded that the ionization 

 of the air took place quite close to the hot electrodes. 



It has long been known that hot bodies possessed the pro- 

 perty of discharging electricity §. ' The experiments of Elster 

 and Geitel || are fundamental, and the subject has been taken 

 up at the Cavendish Laboratory by Prof. Thomson ^f, who 

 showed that the carriers were charged particles, Prof. McClel- 

 land**, Mr. 0. W. Richardson ft, and Prof. Rutherford %%. 

 The result of all these experiments is to show that a hot 

 metal gives off positive ions at a red heat and negative ion> 

 at a white heat, and that this negative current increases 

 very rapidly with further rise of temperature, the potential- 

 difference being much less than is necessary to produce a 

 discharge in the ordinary sense. 



When we pass to the discharge in a vacuum-tube the 

 phenomena become more complicated, and the results more 

 difficult to interpret. Hittorf § § carried out a very extensive 

 series of experiments on the temperature effects at various 

 parts of the discharge. He found that the luminosity in the 

 positive column was extinguished in the neighbourhood of a 

 heated platinum spiral, or if the anode itself were made white 

 hot. On heating up the cathode he found no marked dimi- 

 nution in the total potential- difference until a yellow heat 



* Communicated bv Prof. J. J. Thomson, F.K.S. 



t Phil. Mag. [5] xxix. pp. 358, 441. 



X Phil. Trans, cxcvii. p. 415 (1901). 



§ Guthrie, Phil. Mag. [4] xlvi. p. 257 (1873). 



|| Wied. Ann. xxxviii. p. 27 (1889). 



U Phil. Mag. [5] xliv. p. 203 (1897). 

 ** Proc. Camb. Phil. Soc. x. p. 241 (1900), and xi. p. 296 (1901). 

 ft Ibid. xi. p. 286 (1901). 

 XX Phys. Eev. xiii. p. 321 (1901). 

 §§ Wied. Ann. xxi. p. 90 (1884)., 



