On the Electron Iheory of the Carbon Arc. 301 



increased by the heat of reaction. The small value of ejm 

 recently obtained for the positive ions (384 for platinum) by 

 Professor Richardson, indicates that they are not positive 

 electrons. The balance of evidence appears to be in favour of 

 the view that they are atoms or molecules of some foreign 

 substance emitted from the metal by a process similar to 

 evaporation and involving small thermal changes. The 

 matter can hardly be regarded as definitely settled. 



The author takes great pleasure in acknowledging his 

 indebtedness to Professor Richardson for his interest and 

 advice throughout the course of the investigation. 



XXXIII. A Note on the Electron Theory of the Carbon Arc. 

 By J. A. Pollock, Professor of Physics in the University 

 of Sydney *. 



1. TyrnonrCTIOK.—A general description of the 

 phenomena of the carbon arc, from the point of view 

 of the electron theory, is given by Professor J. J. Thomson 

 in his book on the Conduction of Electricity through 

 Gases f; here it is proposed to discuss merely the questions 

 arising from a consideration of the results of a research 

 on the Resistance and Electromotive Forces of the Electric 

 Arc by Mr. Duddell %. This investigation, carried out with 

 characteristic ingenuity and skill, yielded the important 

 discovery of both a back and forward electromotive force in 

 the arc, and the measurements obtained finally solve the 

 old problem of the nature of the distribution of the potential 

 between the carbons. 



2. Experimental Results, — In the case of a direct current, 

 0*91 ampere arc, 6 millimetres long, between solid Conradty 

 Norris carbons, 11 millimetres in diameter, Mr. Duddell 

 gives the following figures for the arc potential-difference : — 

 At the surface of the anode a back electromotive force of 

 1()*7 volts, followed by a fall of potential of l(i volts within 

 an extremely small distance from the anode surface, then a 

 fall of 25 volts through the vapour column, a fall of 11*7 

 volts within a small distance from the cathode, and a forward 

 electromotive force of 6*1 volts at the cathode surface. 



3. The Electromotive Forces in the Arc. — When an ion of 

 mass m moves freely between two points differing in potential 



* Communicated by the Author. Read before the Electrical Asso- 

 ciation of Xew South Wales. 



+ See also Stark, Ann. dor P/n/si/c, xii. p. 678 (1903). 

 X Duddell. Phil. Trans. A. 203, p. 305 (1904). 



