324 
MRS. H. AYRTON ON THE MECHANISM OF THE ELECTRIC ARC. 
Every increase of current, whether the carbons are solid or cored, entails an 
enlargement of the cross-section of the arc, and a consequent tendency of the P.D. to 
diminish. While the current is so small, with cored carbons, that the volatilised 
surface does not completely cover the core, the increase of cross-section is unaccom¬ 
panied by any change in the specific resistance of the arc. When the current is so 
large, however, that the solid carbon round the core begins to volatilise, each increase 
Current 
Fig. 12. Curves exemplifying the changes introduced into the curves connecting P.D. with current for a 
constant length of arc by coring the positive carbon. 
of' current is accompanied by two tendencies in the P.D., the one to fall , on account 
of the larger cross-section, the other to rise , because of the higher specific resistance 
of the vapour and mist coming from the solid portion of the carbon. The curve 
connecting the P.D. with the current must, therefore, be compounded of two. One, 
such as A B C (fig. 12), which would connect the P.D. with the current if the positive 
