PD between Carbons in Volts. 
FG. 9.—Curves connecting P.D. and Current for nearly Constant Length of Arc of 1°5 mm. to 
—Positive, 11 mm. Solid ; 
304 
NATURE 
| JuLy 27, 1899 
P.D. between the carbons for a given current seems to have 
increased as the length of time during which the arc had been 
burning increased ; this was undoubtedly partly due to the 
lengthening of the arc, but was probably also partly due to the 
whole of the air in the pot having been gradually burnt up or 
driven out through the slag wool and the asbestos ring by the 
pressure of the carbon vapour. 
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HM Ring of sheet Asbestos. 
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Y MI “end Plaster of Paris. ° 
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j-Firectay Pot. 
7--Firectay Cement. 
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*. Sodium Silicate and 
Plaster of Paris Cement. 
Yy | 
a. Wh 
| 
ll}-Lower Carbon. 
Fic. 8. 
Many other sets of curves were obtained, but all with the 
same result, viz. that when once the crucible had been freed 
from air, no sudden diminution in the P.D. could be observed 
on increasing the current far beyond the value at which this 
diminution occurred on lifting up the lid and allowing the air 
to have access to the arc. 
The next thing to do was to try if an open arc could be made | 
to hiss and the P.D. to diminish suddenly by blowing air at the 
ARC ENCLOSED 
lege, suggested using a tubular positive carbon and blowing the 
air down it. This plan answered admirably, for whena current 
of 10 amperes was flowing with an are of about 3 mm., so that 
the arc was quite silent, each puff of air blown down through 
the positive carbon was followed by a hiss and the characteristic 
diminution of the P.D. between the carbons. With a current 
of 6 amperes, however, I could get no hiss, but simply blew the 
arc out each time, probably because, with such a small current, 
the arc was cooled sufficiently to be extinguished before the 
action could take place. 
Oxygen was next tried, still with the open arc, and again 
each puff produced a hiss and diminution of the P.D., the latter 
being exactly the same in amount as when air was used, namely, 
about 10 volts. As my idea was that the diminution of the 
P.D, was due to the chemical combination of air with carbon 
at the temperature of the crater, the fact of oxygen producing 
the same diminution as air seemed to show that nitrogen would 
produce no effect, and that all the effect produced by air was 
due to the oxygen in it. Accordingly I tried blowing nitrogen 
down the positive carbon of an open are, and found that xe 
change in the P.D. followed if the nitrogen was blown through 
gently, but that, beyond a certain pressure, the arc was blown 
to one side, and thus lengthened, so that the P.D. vase as it 
always does when the arc is lengthened, and, if the pressure 
continued, the arc went out. 
This experiment proved two things—firstly, that it is the 
oxygen in the air that causes the diminution in the P.D. with 
hissing ; secondly, that this diminution is not due to cooling, 
for nitrogen would cool the arc as effectually as oxygen or air. 
To make assurance doubly sure on this point, carbon dioxide 
| was blown down the tubular positive carbon, with the same 
result as when nitrogen was used, viz. no change was produced 
in the P.D. between the carbons unless the pressure of the 
gaseous stream were large enough to blow the arc on one side, 
and then an increase and not a diminution in the P.D. was 
observed. 
If, however, the current was very eav the value that made 
an open arc of the particular length used start hissing, blowing 
either nitrogen or carbon dioxide through the positive carbon 
sometimes started hissing ; but this was due, of to any direct 
action of the stream of gas on the carbon, but to the arc being 
deflected by the gaseous stream and burning obliquely up the 
side of the carbon, and thus allowing the air to come into 
contact with the crater. The proof of this was that this 
diminution in the P.D. had the same value as if air had been 
IN CRUCIBLE. 
A 
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a 
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iS 
| 
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22 
agen 
50 38 40 
Current in Amperes 
was varied. Carbons : 
crater, when the current was so small that the crater remained | 
well at the end of the positive carbon—in fact, to bring the air in 
contact with the crater artificially when a much smaller current 
was flowing than would usually produce hissing. I first tried | 
inserting a carbon tube in the arc, and blowing through it. but 
this almost invariably blew the arc out. Then Mr. Phillips, 
one of Prof. Ayrton’s assistants at the Central Technical Col- | 
NO. 1552, VOL. 60] 
2mm. ‘The arrows show the direction in whith the Current 
Negative, 9 mm. Solid. 
employed, and that the hissing did not cease when the stream 
of nitrogen or of carbon dioxide was stopped. 
This was not the case with hydrogen, however. When that 
gas was blown down the positive carbon in the open air, the 
arc would start hissing, if the current were large enough, avd 
slop hissing the moment the hydrogen was shut off, Not only 
this, but the diminution in the P.D. had a different value from 
