MRS. H. AYRTON ON THE MECHANISM OE THE ELECTRIC ARC. 
329 
the first quick swing of the voltmeter needle in the same direction as that of the 
ammeter, but never when both were solid. For as the resistance did not alter 
directly after the current, with the cored carbons, the new current would be flowing 
through the old resistance for an appreciable time after the change, and so the 
accompanying change of P.D. in the same direction as the change of current would 
be able to influence the voltmeter needle. 
The Change in the Specific Resistance of the Arc produced by a Change of Current 
when Either or Both Carbons are Cored. 
We have next to consider SV^/SA, the part of SV/SA that depends on the changes 
in the specific resistances of the mist and the vapour that occur with each change 
of current, when either or both carbons are cored. 
As it is the positive carbon only that volatilises , while the negative simply burns, 
coring the negative carbon alone must have a very different effect on the specific 
resistance of the arc from coring the positive alone. For when the negative carbon 
alone is cored, the whole of the vapour and almost the whole of the mist must issue 
from the uncored carbon, the core in the negative carbon only contributing a little 
metallic vapour to the mist in contact with it; when, on the other hand, it is the 
positive alone that is cored, the whole comes from the cored carbon. Thus, while, 
with the cored negative, the vapour is always solid-carbon vapour, and the mist is 
practically solid-carbon mist, with the cored positive the vapour and mist are both 
core vapour and mist alone, until the current is large enough for the volatilising 
surface to cover the whole core, and they only begin to have an admixture of solid- 
carbon vapour and mist when the current is larger than this. When, therefore, the 
negative carbon alone is cored, the specific resistance of the vapour is constant, and 
that of the mist increases with each small increase of current, but more and more 
slowly, with the same addition of current, the larger the original current before the 
addition is made. The curve connecting SV^/SA with the normal current in this case 
must, therefore, be of the form ABC (fig. 13), for the change of specific resistance 
must be greatest when the current is just large enough for the mist to cover the 
whole core, and must steadily diminish as the direct current increases after that, till 
it becomes practically zero with very large currents, so that the curve becomes 
asymptotic to the axis of current. 
When the positive carbon alone is cored, the curve is quite different. If the arc 
always remained perfectly central, it would be of the form D E F G (fig. 13). The 
specific resistances of the vapour and mist would remain constant till the volatilising 
surface was large enough to cover the core, so that until then SV,/SA would be zero, 
and D E would be the first part of the curve. The first increment of current that 
was added after this would increase the specific resistances by the largest possible 
VOL. CXCIX.—A, 2 U 
