MRS. H. AYRTON ON THE MECHANISM OF THE ELECTRIC ARC. 
301 
must be three sorts of material in the gap, therefore, marking the three stages 
through which the vapour is continually passing. 
1. It starts as a thin film of carbon vapour spread over the end of the positive 
carbon. 
2. It then changes into the mist that lies between this vapour film and the 
negative carbon. 
3. Finally it burns and forms a sheath of burning gases which encloses not 
only the fresh vapour and mist, but also the ends of the solid carbons 
themselves. 
The Conducting Power of the Vapour, Mist, and Flame. 
The specific resistances of true vapours have been shown to be high, therefore 
I conclude that the film over the end of the positive carbon has a high resistance, 
even though it be very thin. The mist, on the contrary, is composed, 1 think, of 
minute solid particles of carbon, and must, therefore, I anticipate, have a lower 
specific resistance. My experiments on the flame have shown, on the other hand, 
that its specific resistance is so high, compared with that of the inner purple mist, 
that it is relatively an insulator, a result confirming that obtained by Luggin # in 
1889. The current, therefore, flows through the vapour and the mist, and practically 
not at all through the sheath of burning gases. 
The Production of the High Temperature at the Crater. 
To explain the great production of heat at the end of the positive carbon, as well 
as the sudden change of potential that is known to exist there, it has been supposed 
that a back E.M.F. of some 35 to 40 volts existed at the junction of the crater and 
the arc. But if, as I suggest, there be a high resisting vapour film in contact with 
the crater, the current passing through this must generate much heat, and this heat 
is utilised mainly in continuously forming fresh carbon vapour, at the lowest tem¬ 
perature at which carbon will volatilise—to be itself turned into mist, and then into 
flame. Hence it seems probable that the high and constant temperature of the crater 
is kept up by the current flowing, not against a back E.M.F., but through the resistance 
of a thin vapour film of constant temperature lying over the surface of the crater. 
In other words, it is not the crater itself that is the source of the heat of the arc, hut 
a thin film of carbon vapour, at constant temperature, in intimate contact with it. 
* ‘ Wien. Sitzungsberichte,’ vol. 98, p ? 1, 233. 
