410 



Mrs. H. Ayrton. 



"The Mechanism of the Electric Arc." By (Mrs.) Hertha 

 Ayrton. • Communicated by Professor Perry, F.RS. 

 Received June 5, — Read June 20, 1901. 



(Abstract.) 



The object of the paper is to show that, by applying the ordinary 

 laws of resistance, of heating and cooling, and of burning to the arc, 

 considered as a gap in a circuit furnishing its own conductor by the 

 volatilisation of its own material, all its principal phenomena can be 

 accounted for, without the aid of a large back E.M.F., or of a " negative 

 resistance," or of any other unusual attribute. 



The Apparent Large Bach E.M.F. 



It is shown how volatilisation may begin, even without the self- 

 induction to which the starting of an arc, when a circuit is broken, is 

 usually attributed ; and it is pointed out that, when the carbons are 

 once separated, all the material in the gap cannot retain its high 

 temperature. The air must cool some of it into carbon mist or fog, just 

 as the steam issuing from a kettle is cooled into water mist at a short 

 distance from its mouth. The dissimilar action of the poles common 

 to so many electric phenomena displays itself in the arc at this point. 

 Instead of both poles volatilising the positive pole alone does. It is 

 considered, therefore, that the arc consists of (1) a thin layer of 

 carbon vapour issuing from the end of the positive carbon, (2) a bulb 

 of carbon mist joining this to the negative carbon, and (3) a sheath of 

 burning gases, formed by the burning of the mist, and the hot ends of 

 the carbons, and surrounding both. The vapour appears to be indicated 

 in images of the arc by a sort of gap between the arc and the positive 

 carbon, the mist by a purple bulb, and the gases by a green flame. 



The flame is found to be practically insulating, so that nearly the 

 whole of the current flows through the vapour and mist alone. It is 

 suggested that the vapour has a high specific resistance compared with 

 that of the mist, and that it is to the great resistance of this vapour- 

 film that the high temperature of the crater is due, and not to any 

 large back E.M.F. of which it is the seat. 



Volatilisation can only take place at the surface of contact between 

 the vapour film and the positive carbon. When that surface is smaller 

 than the cross-section of the end of the carbon, it must dig down into 

 the solid carbon and make a pit. The sides of the pit, however, must 

 be hot enough to burn away where the air reaches them, hence there 

 is a race between the volatilisation of the centre of the carbon and the 

 burning of its sides that determines the shape of the carbon. When 

 the arc is short, the air cannot get so easily to the sides of the 



