282 
NATURE 
[JuLy 20, 1899 
ELECTRIC ARC) 
ISSING is one of the few phenomena connected with the 
electric arc with which every one is more or less familiar. 
In the old days the sudden, almost complete, extinction of the 
light of an arc-lamp, and the loud hiss accompanying its 
relighting, was socommon an occurrence that it was supposed 
by the lay mind to form part and parcel of the working of the — 
““ electric light,” and led to a lively prejudice against that light 
on the part of the public. In these days of .enclosed arcs and | 
of better constructed lamps, such little interludes are of far less | 
frequent occurrence ; but it is as important as ever, from a 
scientific point of view, to discover their origin, and even from 
the practical side anything which points to a remedy for this | 
grave defect in arc-lighting cannot fail to be of interest. | 
The object of the present article is to explain the cause of | 
hissing in direct current open arcs; that is to say, in arcs in 
which the current flows always in one direction, and to which | 
the air has free access at every point. | 
{ 
THE REASON FOR THE HISSING OF bi) 
| 
a sound something like that of steam under pressure, issuing 
from a pipe. This sound is accompanied by a diminution of 
about ten volts in the P.D., or electric pressure between the 
carbons, and an increase in the current. 
For the experiments on which the present article was based, 
three sets of electrical measurements were made, viz. measure- 
ments of the current, the P.D. between the carbons, and the 
length of the arc. Before each observation was made the 
current and length of are were kept 7%gorvozs/y constant for 
a sufficient length of time for the carbons to take their character- 
istic shape for that particular current and length of arc, and 
long enough, therefore, for the P.D. to have become constant 
also. Such an arc is called a zovmal arc, as contrasted with 
one arrived at in a haphazard fashion by suddenly giving the 
current some particular value and the arc some particular length, 
and making observations without allowing time for the carbons 
to acquire their proper forms. 
The carbons used were generally both so/d, that is, neither 
had a soft core such as is usually given to the positive carbon 
when the arc is employed for lighting purposes, and they were, 
OPEN ARCS. 
Sotia Carvors 
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Ss 
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Ss 
PD between carbons in Volts. 
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Current in Amperes. 
Fic. 1.—Curves connecting P.D. and Current for Constant Length of Are. 
Carbons :—Positive, 11 mm. ; Negative, 9 mm. 
There are other ways in which a change taking place in the 
arc may manifest itself, in addition to giving out sounds of 
various kinds or by becoming silent. For example, there may 
be changes in its electrical measurements, or an alteration in 
the appearance of the crater, the arc, and the carbons. 
The sounds given out by the direct current open arc are many 
and various, but only two seem to possess much significance — 
the hum and the hiss—and the causes of these are evidently 
connected with one another, for the hum never occurs except 
when the arc is on the point of hissing or has just been hissing, 
although it is quite possible to make an arc hiss and become 
aie again without any hum being heard either before or 
alter. 
The hum is a distinct musical note, which is often quite low 
to start with, and gets higher and higher, till it finally rises to 
a shriek, and then the arc breaks into a loud hiss, giving forth 
1 Based on a paper read before the Institution of Electrical Engineers 
by Mrs. W. E. Ayrton. 
NO. 1551, VOL. 60] 
as usual, placed vertically over one another with the positive 
carbon on top. 
Some of the results of these experiments are given in Fig. 1, 
in which the curves connect the P.D. between the carbons 
with the current, for various constant lengths of arc. Starting 
from the left, each curye goes smoothly on its way, as the 
current increases, till a certain point is reached, when it suddenly 
breaks down, and is continued in a straight line far below and 
to the right of its own lowest point.. The break-down occurs 
when the current has such a value that the arc can no longer 
remain silent. The dotted lines, which join the cwsves for silent 
arcs to the strazght /énes for hissing arcs of the same length, 
indicate ranges of current that will not flow through the par- 
ticular length of arc indicated at all if the arc is normal with 
the arrangement of the circuit existing when the experiments 
were made. 
An examination of these curves shows that with the carbons 
used, and with the xorvma/arc, the following results are met with: 
