MRS. H. AYRTON ON THE MECHANISM OF THE ELECTRIC ARC. 
321 
There are two ways in whieh the P.D, between the carbons, for a given current 
and length of arc, may be lowered by the core ; (1) by an increase in the cross- 
section of the vapour film, or the mist, or both ; (2) by a lowering of their specific 
resistances. To see whether I could observe any change in the cross-sections, I have 
traced a series of enlarged images of the arc with four sets of Apostle carbons, 
using (1) + Solid — Solid, (2) + Solid — Cored, (3) + Cored — Solid, (4) + Cored 
— Cored carbons. 
The positive carbon was 11 millims. and the negative 9 millims. in diameter, and the 
arc 2 millims. in length in each case, while the currents were 4, 6, 8, 10, 12, 14 amperes. 
The diagrams were traced not only when the arc was normal in each case, but also 
immediately after each change of current, so that the effect on the cross-section of the 
arc of both an instantaneous and a normal change of current might be seen. Fig. 5 (p. 307) 
showed the first set of diagrams of the normal arc; the others are too numerous to 
publish, but the mean cross-sections of the purple part—the mist—in each, measured 
as in fig. 5, may be found in Table III., those marked “non-normal” belonging to the 
arc immediately after the change of current, and those marked “ normal ” to the arc 
after all the conditions had become steady again. 
Table III.—Mean Cross-Sections of Mist between fi- Solid — Solid, + Solid — Cored, 
-f Cored — Solid, and -f Cored — Cored Apostle Carbons, 11 millims. and 
9 millims. 
Length of Arc, 2 millims. 
Current in 
amperes. 
Normal. 
Non-normal. 
S.S. 
S.C. 
C.S. 
C.C. 
S.S. 
S.C. 
C.S. 
C.C. 
4 
4-8 
6-95 
4-0 
3-3 
6 
9-8 
8-3 
6-05 
5 • 6 
9-5 
. 8-4 
6-25 
3-5 
8 
16-2 
14-2 
11-0 
8-9 
17-6 
11-1 
12-0 
5-8 
10 
23-4 
20-75 
13-55 
11-9 
21-5 
19-0 
18-7 
11-1 
12 
34-9 
27-6 
17-7 
16-55 
34-1 
26-9 
20-1 
16-7 
14 
41-2 
35-0 
24-5 
20-0 
— 
39-4 
— 
18-7 
With a single exception, every number in each set is smaller than the corresponding 
number in the preceding column. Hence, with both normal and non-normal arcs the 
mean cross-section of the mist, for a given current, is largest when both carbons are 
solid, smallest when both are cored, and is more diminished by coring the positive 
than by coring the negative. Fig. II, besides showing well this marked difference in 
the influence of the cores, makes it apparent that the difference increases, in every 
case, with the current, for such currents as were there used, 
yol. cxcix,— a, 2 t 
