Sept. 19, 1878] 



NATURE 



547 



ELECTRIC DISCHARGE IN GASES ^ 



II. 



'T^HE form of point most favourable to the production of 



■■■ the arc has been minutely investigated by the authors. 



By turninsT wires in a lathe to various outlines they arrived 



experimentally at the best point ; this was then placed 

 under the microscope and drawn by means of the camera 

 lucida ; from the study of the drawing it was ascertained 

 that the longest spark was procured when the point 

 assumed a form resembling a paraboloid ; the curved out 



Fig. 8. — "Streamers." 



line, which corresponded to that found experimentally, 

 was one in which each succeeding ordinate was in the 

 ratio of the square root of the odd numbers i, 3, 5, &c., 

 the sectional areas being consequently in the ratio of the 

 odd numbers. 



The curves in the diagram (p. 528) show the distances at 

 which, with a given potential, the arc is formed between 

 such a point and a disc, and between two such points respec- 

 tively. The results recorded in the case of the point and 

 ■disc are those obtained by electrifying the point to the 

 sign (positive or negative) which gave the greatest length ; 

 for it was found that with low potentials the distance at 



Fig. 9 — '■ Glov,-.'' 



which the arc forms is greater when the point is negative, 

 that between 3,000 and 5,000 volts it is the same whether 

 the point be positive or negative, but that with potentials 

 higher than 5,000 volts it is greater when the point is 

 positive. 



The actual formation of the arc, when a point and disc 

 or two points are employed as terminals, is preceded by a 

 luminous discharge (streamers and glow), presenting 

 phenomena of an interesting character; an extremely 



* Cont'niied from p. 52S. 



minute quantity of electricity passes as compared to that 



fit;, i^. — "otrcauierj aud Glow, 



when the arc is formed, but still it -s su'ficient when 



