224 O. N. Rood on the Electric Spark. 
in contact with it, and their path is marked by & 
comet-like tails (see fig. 3). The diameter of _ = 
these tails is the same with that of the circles. 3 
This is also true of the brush. = 
Electrical brushi_When owing to distance or the use of 4— 
inted wire these partial sparks become reduced in intensity we 
ave the electrical Gros: its form is slightly removed from the 
last; the middle circle of dots vanishes as well as the projections 
from the broad inner ring, and we have two concentric rings, the 
smaller one being most strongly marked. If the brush be still 
farther reduced in intensity the external circle becomes very 
faint and finally disappears, and but a single circle is left— 
Fig. 2, IT, IV. 3 
‘lectricians have long since arrived at the conclusion that the 
electrical spark passes by insensible gradations into the electrical 
brush, and I find that photography furnishes a beautiful confirm: 
ation of this view: thus the figures I, IT, III, IV, Fig. 2, are s* 
lected from photographs form the first members of a series which 
would illustrate the gradual conversion of one of these forms 
into the other, and the other members could easily be added. 
e have seen that while the electrical brush is characterized 
py the annular form, electricity of higher tension generates stat 
like figures, the rays being larger in proportion to the tensio? 
up to a certain point ; now the constant occurrence of a combi- 
nation of these two forms, in the photographs of bright sparks, 
points out, I think, clearly, that these sparks consist also of - 
than one discharge; moreover, the ring where it cuts the rays° 
the star can often be traced under them, as though superpositio 
had taken place. Again, when the bright spark travels goes 
distance the ring is generally not symmetrically placed, as oer 9 
the discharge producing it had followed a slightly different pat ‘ 
Indications are constantly obseryed, which lead to the idea tha 
even the star itself, is produced by the overlaping of two sta™ 
having rays of different size and different intensity. 
The wet collodion film offers of course a certain amount of T& 
sistance to the passage of electricity over its surface, aD 
ishes us with the different indications above decribed Fl 
Finally, as the researches of Riess, Kirchhoff, Helmholtz a™ 
Feddersen, have shown that the electrical discharge is oscillatotY 
and wave-like, Iam the more inclined to regard these ge 
graphic figures as produced by a series of consecutive dischar 
of different intensi 
: : Pp : 
in ‘on ification which the positive partial spark oer 
them hem fall quickly on the same spot: see V, fig. 2. 
: aangd _— ged radially around the point under 
being turned outward. 
