322 
MRS. H. AYRTON ON THE MECHANISM OF THE ELECTRIC ARC. 
We cannot measure the cross-section of the vapour film directly, hut, for a constant 
length of arc, it must he roughly proportional to the cross-section of the mist where 
it touches the crater. These cross-sections, which are given in Table IV., do not, 
naturally, vary nearly so regularly as the mean cross-sections, hut still we can judge 
pretty well what are the effects of the various cores. Coring the positive carbon, 
for instance, distinctly diminishes the cross-section of the vapour film ; for every 
Fig 11. Curve connecting the mean cross-section of the arc mist with the current for + solid, - solid, 
-f solid - cored, + cored - solid, and + cored - cored carbons respectively. Apostle carbons, 11 millims. 
and 9 millims. Length of arc, 2 millims. 
number in column (7) is less than the corresponding one in column (5), and all hut 
one in column (3) are less than those in column (1). Coring the negative carbon, on 
the other hand, only seems to affect the cross-section that the vapour film 
assumes immediately after a change of current, for while in the non-normal section 
each number in (6) is less than in (5), and in (8) less than in (7), in the normal 
section the numbers in (2) are sometimes less and sometimes greater than those in (1), 
and those in (4) are nearly all greater than those in (3). 
