334 
MRS. H. AYRTON ON THE MECHANISM OF THE ELECTRIC ARC. 
of alternating current for which the curves are drawn, but their relative shapes and 
positions must he similar to those in fig. 16 whatever the length of the arc and the 
frequency. 
Fig. 16. Suggested curves connecting SV/SA with the direct current for a constant length of arc. 
The Effect Produced by Coring Either or Both Carbons on the Curve connecting the 
Non-Normal Value of SV/SA with the Length of the Arc when A is Constant. 
Finally, we come to the curve connecting SV/SA with l, the length of the arc, when 
the frequency of the alternating current and the value of the direct current are both 
constant. 
We must refer first of all to the connection between SV/SA and l when both 
carbons are solid, in order to see how this connection is varied by the cores. P Q 
(fig. 17) is the rise of P.D. that would accompany the increase of current SA, with 
an arc of l millims., if the resistance of the arc did not alter with the current. Q It 
is the fall of P.D. due to the enlargement of the vapour film and the mist (the 
frequency of the superimposed alternating current is taken too great for the carbons 
to be able to change their forms). When the current increases from A to A + SA, 
therefore, the P.D. actually falls from P to 11. Now the rise P Q depends only on 
the amount by which the current is increased, and the resistance through which that 
increased current has to flow, i.e., on SA, A, and l ; or, since A and SA are supposed 
to be the same for each length of arc, P Q depends simply on l, and increases when 
l increases. 
