O. N. Rood on the Electric Spark. 365 
(a.) When a brass ball is used as positive electrode and the 
moist plate is connected with the negative pole of the coil, the 
figure consists of a central thick and irregularly shaped ring, 
and an external portion formed like an irregular star; see 
which is enlarged three diameters, as also are all the other draw- 
ings of the spark in this number, except where 
the contrary is expressly stated. The central 4 
portion within the first mentioned ring is more 
or less blackened, sometimes being quite dark, 
so that the ring can hardly be distinguished. In 
One case 14 sparks generated under the above 
mentioned circumstances were allowed to fall on 
the sensitive plate, which was then somewhat 
hereafter simply by the expression ‘‘ washed plate.” In 
these fourteen cases, the interior of the above mentioned ring was 
uniformly darkened, and of a purplish color by transmitted light. 
It often happens that two of the external starlike figures are 
produced and superimposed; this was very beautifully shown in 
this washed plate, as well as in some which were unwashed. This, 
and other indications, obtained in positive and negative figures,” 
point out that it is a matter of chance whether single, double, or 
triple sparks are obtained by the ordinary manipulation. 
(6) When the positive electrode is a metallic point, a figure 
like 5 is obtained, its interior being often more or e 
ess shaded; it is also generally surrounded by a 
diffused faint mass of shade. 
(c.) When two points are used, as before de- 
scribed, the figure remains essentially the same as 
With a single point. - 
and the so-called luminous “atmosphere,” and it will be shown 
at the conclusion of this article, that when the effects produced by 
the two different portions are pe amet the aecrggehuig tears 
the figure generated by frictional electricity 1s much closer 
it here appears. 
: * Compare figure 22. 
