226 O. N. Rood on the Electric Spark. 
If the sensitive plate be placed on the prime conductor charged 
with positive electricity, and sparks be drawn from the sur 
face of the plate, the negative figure is produced; if the plate 
rest on a conductor charged with negative electricity and sparks 
be drawn from it the positive form is obtained ; and finally, 
the plate be coated on both sides with collodion and insulated 
with a brass ball before and behind it, one of them being also 
insulated, while the other is in communication with the ground— 
if now a spark of either kind of electricity be allowed to pass 
from the insulated ball to the plate and from the plate to the 
second ball, we obtain as would be expected, on the opposite sides 
of the plate, the negative and positive images by development. 
Tt is well known that when the knob of a jar, charged with 
positive electricity, is touched to a thin plate of pitch, a yellow 
star-like figure is produced by sprinkling the plate with a mu 
ture of powdered sulphur and red lead; while if negative elec- 
tricity is used a rounded red figure is formed. These fg 
bear the name of their discoverer, Lichtenberg. The meth 
employed by Riess is much better calculated to give accurate 1 
sults than the common one just mentioned: asmall square plate 
of copper is coated thinly with black pitch on one side 
point of an insulated metallic rod touches the centre of the pit 
surface, while the other side of the plate is in metallic eee 
tion with the ground. If now a spark from a jar charged wit 
positive electricity be allowed to pass over to the pt inted om 
and the latter still insulated be removed, then by sprinkling #° 
pitch with a mixture of sulphur and red lead the star 18 et 
rated in great purity: very perfect red negative figures até 
course formed in a corresponding manner. cy 
In repeating these experiments after the manner Of Riess, : 
was struck with the resemblance existing between the red ~~ 
tive disc and the photographs of the negative spark. There 
also much general resemblance between the positive yellow st 
and the photographs of the positive spark : this is greatl heig: 
ened if the yellow positive figures are produced in the follow 
uction. 
The results obtained with Leyden jars, and in a partial ee 
e 
as well as by the use of metallic plates, I propose to d 
fature occasion. : 
