618 Mr. N. R. Campbell : Experiments on 



produce large currents when the potential is sufficiently high 

 to cause new ions to be generated in sufficient numbers I y 

 collisions. In this case the sparking could not take place 

 unless the quantity a—jBe-"'~^^^ yanished, and possibly it 

 might require a slightly larger potential than that which 

 corresponds to this condition in order to ensure the deyelop- 

 ment of continuous currents from such a small number of 

 ions. There are a number o£ points in connexion with these 

 subjects which require further inyestigation, and I hope to 

 make some othpr experiments in this direction. 



The experiments which I haye giyen on sparking potentials 

 w^ere made with a source of ultra-yiolet light of small in- 

 tensity, which only reduced the sparking potential very 

 slightly below that which was obtained when no external 

 radiation acted on the negatiye electrode, and the agreement 

 between these potentials and the potentials found by calcu- 

 lation shows that a satisfactory explanation of the sparking 

 potentials in gases may be obtained from the theory of the 

 genesis of ions by collision. 



LXX VII. Some Experiments on the Electrical Discliarge from 

 a Point to a Plane. By Noeman R. Campbell, B.A.^ 

 Scholar of Trinity College^ Camhridge*. 



AMONG the many inyestigations on the discharge from a 

 point are those of Warburg! and Tammj, who haye 

 measured the relation between the current and the potential- 

 ditterence : the production of ozone and other chemical 

 reactions produced by the discharge haye been studied by 

 Warburg §, Cook 1 1, and seyeral others; but no researches appear 

 to haye been made with the object of elucidating the mechanism 

 whereby the current is transferred between the electrodes. 

 It was with the hops of throwing some light upon this 

 problem that the following experiments were undertaken. 



The principle of the method adopted depends on the 

 results obtained by Mr. C. T. R. Wilson in his work on the 

 Condensation Nuclei produced in gases by yarious agents If : 

 these experiments are, in fact, an extension of those described 

 in pp. 441-444 of that paper. By producing a known 

 adiabatic expansion in a vessel through which the discharge 



* Communicated bv Prof. J. J. Thomson. 

 t E. Warburg, Wied. Ann. Ixvii. p. 72 (1899). 

 X F. Tamm, An. Phys. vi. p. 259 (1901). 

 § E. Warburg, Berl Ber. 1900, pp. 712-721. 

 II E. H. Cook, Phil. Mag. [51 xlvii. p. 40 (1899). 

 «: C. T. R. WHson, Phil. Trans. 192 A. pp. 4Uo-45G (1899). 



