310 
MRS. H. AYRTON ON THE MECHANISM OF THE ELECTRIC ARC. 
when the P.I). between the carbons is changed—increased, say—th e, first result must 
be an increase of current, while the second is a corresponding increase in the cross- 
sections of the vapour film and the mist, causing a diminution of the resistance, and, 
consequently, of the P.D. between the carbons. Thirdly, if the new current is kept 
constant long enough, the end of the negative carbon burns away to a longer 
slenderer point, allowing more of the mist to escape, so that it takes a smaller cross- 
section, and, consequently, both the resistance and the P.D. increase again, although 
they never reach such high values as they had with the smaller current. 
Fig. 7 is useful as showing at a glance how the resistance and the P.D. depend 
upon the time that has elapsed after a change of current. When the arc is normal, 
c 
Time. 
Fig. 7. Suggested simultaneous time-changes of P.D., current and resistance. 
in the first instance, A B, A' B', and A" B" represent the curves connecting the P.D., 
the current, and the true resistance of the arc respectively with the time. When 
the P.D. is increased from B to C, the resistance does not alter at the first instant, 
but the current rises to C'. If it is then kept constant at C', the surface of volatilisa¬ 
tion next increases in area, the resistance falls to I)' , and the P.D. consequently 
falls to D. After this the carbons begin to grow longer points ; the cross-section of 
the mist diminishes, the resistance, therefore, increases to E", and the P.D. with it 
to E. The arc has now become normal again, so that the curves are all now parallel 
straight lines, the current higher than before, and the P. D. and resistance lower. 
Thus any alteration that is made and maintained in the arc sets up a series of 
changes in its resistance and, consequently, in the P.D. between the carbons, that 
