320 
MRS. H. AYRTON ON THE MECHANISM OF THE ELECTRIC ARC. 
The Changes Introduced into the Resistance of the Arc by the Use of 
Cored Carbons. 
Next let us consider the explanation of the marked effects produced by introducing 
a core into either or both of the carbons. These are of a two-fold character ; first, 
those such as Professor Ayrton published at Chicago in 1893, viz. : 
(1.) The P.D. between the carbons is always lower for a given current and length 
of arc, when either or both of the carbons are cored, than wdien both are 
solid. 
(2.) With a constant length of arc and increasing current, the P.D., which 
diminishes continuously when both carbons are solid, either diminishes less 
when the positive is cored, or after diminishing to a minimum remains 
constant over a wide range of current, or even increases again.* 
(3.) It requires a larger current with the same length of arc to make the arc 
hiss when the positive carbon is cored than when both are solid. 
Secondly, there are the facts connected with the influence of cores on the small 
change of P.D. accompanying a small change of current, to which attention was 
first drawn by Messrs. Frith and Rodgers in 189 6. t These facts, which were 
physically correct, although, as I have already shown, they were wrongly interpreted 
at the time, are embodied in the following wider generalisations which I have deduced 
from the results of my experiments, and from theoretical considerations. 
(1) When, on a direct current arc, an alternating current is superimposed which 
is small, but yet large enough for the resistance of the arc to be altered 
by it, the average value of SV/SA is always more positive! when either 
carbon is cored than when both are solid, and most positive of all when both 
are cored, all other things being equal. 
(2) The frequency of the alternating current that makes SV/SA begin to have 
a positive value is lower when either carbon is cored than when both are 
solid, and lowest when both are cored. 
(3) The value of SV/SA, with a given root mean square value of the superimposed 
alternating current, depends not only on the nature of the carbons and on 
the frequency of that current, but also on the magnitude of the direct 
current, and on the length of the arc. 
* Nebel observed the fact that the P.D. fell to a minimum and then rose again, in 1886, but as he used 
cored positive carbons only, he did not discover that this form of curve was peculiar to those carbons. 
t “The Resistance of the Electric Arc,” ‘Phil. Mag.,’ 1896, vol. 42, p. 407. 
I I call SY/SA more positive in one case than in another when it has either a larger positive value, or 
a smaller negative value in the first case than in the second. 
