MRS. H. AYRTON ON THE MECHANISM OF THE ELECTRIC ARC, 
307 
solid carbons, the whole resistance of the normal arc diminishes more rapidly than the 
current increases, and consequently the P.D. must diminish as the current increases. 
Thus, if, in the normal arc, SY be a change made in V, and SA he the corresponding 
Fig. 5. Diagram of 2-millim. normal arc between solid carbons, positive 18 millims. and 
negative 15 millims. in diameter. 
change produced in the current, —, has a negative value, even although the resistance 
of the arc is positive, simply because that resistance diminishes faster than the 
current increases, and vice versd. 
2 E 2 
