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V. The Mechanism of the Electric Arc. 
By (Mrs.) Hertha Ayrton. 
Communicated by Professor J. Perry, F.R.S. 
Received June 5, 1901—Read June 20, 1901. 
Ever since the first discovery of the Electric Arc, nearly 100 years ago, the secret 
of its mechanism has been one of the most fascinating mysteries of science. To 
account for its abnormally high temperature, and for the fact that a higher P.D. is 
required to send a small current than a large one through it, the arc has been 
endowed with unique properties, such as a back E.M.F. of many volts, and even a 
negative resistance. The measurement of this resistance alone has been the object 
of a large number of experiments, made under all conceivable conditions. 
The object of the present paper is to see how far this peculiar behaviour of the 
arc might have been logically predicted from the known conditions of its existence, 
viz., that it is a gap in a circuit, furnishing its own conductor by the evaporation of 
its own material; and to show that it is quite unnecessary to invoke the aid of a 
negative resistance, or even of a large back E.M.F., to account for this behaviour. 
What happens on making the Gap. 
The usual explanation given for the formation of a spark or flash, on opening an 
electric circuit, is that it is caused by self-induction. The interesting question 
therefore arises, could an arc be struck and maintained if there were no self- 
induction whatever in the circuit ? I think it could. For the surfaces of all solids 
are irregular, and therefore all parts of the carbons cannot be separated at the same 
instant. The parts that remain in contact will still conduct the current, but the 
fewer of them that remain the greater will be their resistance. The heat caused by 
this resistance must, at last, be great enough to volatilise the carbon at the remaining 
points of contact, and, by the time that no part of one carbon is touching any part 
of the other, the small gap will be full of carbon vapour. [As the carbon points at 
each junction must volatilise as soon as they are hot enough to do so, this vapour will 
be given off at a constant temperature, viz., the lowest at which carbon can volatilise. 
—March 23, 1902.] 
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