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MRS. H. AYRTON ON THE MECHANISM OF THE ELECTRIC ARC. 
The vapour film, besides radiating heat in all directions from its free surface, must 
lose a certain extra amount of heat all round its edges by conduction through a ring 
of the solid carbon that it does not actually touch. The heat thus lost must be 
subtracted from the edges of the part of the solid carbon that the vapour film 
does touch, and this part will, therefore, be just below the temperature of 
volatilisation, as will also the small ring of solid carbon outside the vapour film. 
Suppose, for instance, that the full line in fig. 3 is the part of the positive 
carbon that is in contact with the vapour film, then the inner dotted line will 
enclose the area that is actually volatilising fresh carbon, and the 
space between the two dotted circles will be at a temperature 
just below that of volatilisation, because the conduction of heat 
from the edges of the vapour film will bring the outer circle 
up and the inner circle down to a temperature a little below 
that of the vapour film itself. The slightly lower temperature 
of the space between the dotted circles would make it perhaps 
a little less brilliant than the volatilising surface, but it would still 
be very much more brilliant than the remainder of the positive carbon, so that it 
must form the outer circle of what we are accustomed to call the crater, viz., the 
most brilliantly white part of that carbon. The area of the crater is thus rather 
larger than the cross-section of the vapour film, while the actively volatilising surface 
is slightly smaller. 
When the carbon vapour proceeds from a given area, the cross-section of the 
vapour film will be greater the more it is protected from the cold outer air by the 
end of the positive carbon. If, for instance, AB, fig. 4, were the diameter of the 
Fig. 3. 
Fig. 4. 
Positive carbons having the same area of volatilisation. C-ABD with a long are. 
C'ABD' with a short are. 
volatilising surface, the cross-section of the vapour film would be greater if the end 
of the carbon were CD, than if it were CTT, or, since the end of the positive carbon 
is thicker the longer the arc, the cross-section of the vapour jihn is greater the longer 
the arc. This film will also be able to keep a larger ring of solid carbon at a 
temperature just below the lowest at which volatilisation can take place, when the 
end of the carbon is CD, than when it is CTV, therefore the whole space that is just 
below the lowest temperature of volatilisation, i.e., that included between the dotted 
circles in fig. 3, will be greater with a long arc than with a short one, when the 
