330 
MRS. H. AYRTON ON THE MECHANISM OF THE ELECTRIC ARC, 
amount, because this would be the point at which the specific resistances of the 
existing vapour and mist and of those added would be most different. Therefore the 
curve would rise suddenly at E. After this, each addition to the normal current 
would make the change of specific resistance due to the same added small non-normal 
increment of current smaller and smaller, so that the curve would fall towards the 
axis of current as shown in F G (fig. 13). Finally, there would already be so much 
solid-carbon vapour and mist in the arc that the addition of a little more would 
make practically no change, so that this curve also is asymptotic to the axis of 
current. The fact that the arc is never really quite central, and that the volatilising 
surface must therefore cover a little solid carbon long before it is larger than the 
core, must introduce some modifications into the first part of the curve, shortening 
D E, and making E F rise less abruptly, something like D E' F' G, but these 
modifications are unimportant. 
Fig. 13. Suggested curves connecting the part of SY/3A that depends on the change in the specific 
resistance of the arc with the direct current for a constant length of arc. 
When both carbons are cored, the curve must be like D E H K, or rather 
D E" H' K, because the effect of the metallic vapour from the negative core will be 
added to that of the positive core, and the change of specific resistance, when solid- 
carbon mist begins to be added, will, therefore, be greater. 
How the Whole Value 0 /SV/ 8 A is Affected by Coring Either or Both Carbons. 
By combining the two changes in the resistance of the arc introduced by the core, 
viz., that due to the changes in the cross-sections of the arc, and that produced by 
the alterations in its specific resistance, we can see how the complete value of SV/SA 
is affected by the core. 
From what has been said on p. 329 it is clear that, if the cross-section ratios in 
Tables V. and VI. can be considered typical, 8 V e /8 A never has a greater negative 
value when the positive carbon alone is cored than when both are solid ; never a 
greater negative value when the negative alone than when the positive alone is 
