JuLy 20, 1899] 
NATURE 
283 
(1) When the length of the arc is constant and the arc 
is silent, it may be made to hiss by increasing the current 
sufficiently. 
(5) For the hissing arc the P. D. is constant for a given length 
of arc, whatever the current. 
It was Niaudet (La Lumiére Electrique, 1881, vol. ili. p. 287) 
OPEN ARCS. 
Positive Carbon Cored. 
- 
ee ape 
fal oie 
wn 
°o 
+ 
or 
a 
a 
; \ 
| 
ea | 
PD. between Cartons in Volts. 
© 
o O +o) 
iF 
+ 
3 
ie [ 
| 
0) TOY TO 
fo) 
, 
“2 
; ee 
tno 
Pe ee | ee 
30 
. 
‘. my 
Sis 
. 
. . D oO 
; : eT 
SSIEENG ———=_== ae 5 
;. o 
Sess 
Soh s 
sats 
suis 
a 
10 12 14 16 
18 20 22 24 26 28 30 32 34 
Current tn Amiperes. 
Fic. 2.—Curves connecting P.D. and Current for Constant Lengths of Arc. 
Carbons :—Positive, 9 mm. Cored. ; Negative, 8 mm. Solid. 
(2) When the current is constant and the arc is silent, 
shortening the are will make it hiss. 
(3) The largest current that will maintain a sz/ez¢ arc is 
greater the longer the arc. 
who, in 1881, first observed the fall of about 10 volts in the 
P.D. between the carbons at the moment hissing began, and, 
although perhaps there is even yet a lingering notion that it is 
only when an arc is short that it can hiss, I find that as far back 
OPEN ARCS. 
Positive Carbon Cored. 
SS 
A 
& mm 
5 mm 
4m 
5am 
— { 
2mm 
| Hissing Unstable State! 
\ mm. LN Se 
mm . fs 
1 | 
{ 
15 * 20 25 30 35 40 
Current in Amperes. . 
Fic. 3.—Curves connecting P.D and Current for Constant Lengths of Arc 
Carbons :—Positive, 18 mm. Cored. ; Negative, 15 mm. Solid. 
t (4) When the arc begins to hiss, the P.D. suddenly falls | as 1889 Luggin (Wen Sztzungsberichte, 1889, vol, xlvii. p- 118) 
about ro volts and the current suddenly rises two or three | showed that, however long an arc might be, it would still hiss 
amperes. 
NO. 1551, VOL. 60] 
were the current increased sufficiently. 
