414 



The Mechanism of the Electric Arc. 



come from the cored carbon. When the; negative, they come from the 

 uncored carbon, and it is only because the metallic salts in the core 

 have a lower temperature of volatilisation than carbon that the mist is 

 able to volatilise these and so lower its own specific resistance. 



The effect of a core in either carbon, or in both, must depend on 

 the current, because the larger the current the more solid carbon will 

 the volatilising surface cover, and the less therefore will the specific- 

 resistances of the mist and vapour be lowered. The way in which the 

 core acts in each case is traced, and the alterations in the specific 

 resistances and cross-sections due to the core are shown to bring about 

 changes in the P.D. exactly similar to those found by actual measure- 

 ments of the P.D. between the carbons. It is shown, for instance, how 

 these changes entirely account for the fact established by Professor 

 Ayrton* that, with a constant length of arc, while the P.D. diminishes 

 continuously as the current increases, when both carbons are solid, it 

 sometimes remains constant over a wide range of current, or even 

 increases again, after having diminished, when the positive carbon is 

 cored. 



The alterations in the value of 8V/8A introduced by the cores are 

 next discussed, and it is shown that the changes in the resistance of 

 the arcs that must follow the observed changes in its cross-section, 

 coupled with the alterations that must ensue from the lowering of its 

 specific resistance, would modify 8V/&A just in the way that Messrs. 

 Frith and Eodgersf found that it was modified by direct measure- 

 ment. Thus all the principal phenomena of the arc, with cored and 

 with solid carbons alike, m&y be attributable to such variations in the 

 specific resistances of the materials in the gap as it has been shown 

 must exist, together with the variations in the cross-sections of the arc- 

 that have been observed to take place. Hence it is superfluous to 

 imagine either a large back E.M.F. or a "negative resistance." 



* Electrical Congress at Chicago, 1893. 



f " The Resistance of the Electric Arc," ' Phil. Mag.,' 1896, vol. 42, p. 407. 



