O. N. Rood on the Electric Spark. 361 
Art. XXXVI.—On the study of the Electric Spark by the aid of 
Photography ; by Prof. OcpEn N. Roop, of Columbia College. 
Il. ON THE FoRM OF THE FIGURE PRODUCED BY THE INDUCTION COIL.* 
In the March number of this Journal for 1862, I described a 
new method of studying the electric discharge by allowing it to 
fall directly on a sensitive photographic plate, where it produced 
___ latent images of certain forms, which, under the action of the 
4 developer, yielded very fine and sharp characteristic pictures. It 
: was shown at that time that there was a very marked difference 
.. between the positive and negative figures, the former consisting 
| essentially of one or more stars and rings in combination, while 
2 the latter was made up, for short discharges, of a collection of 
| : dots or minute circles, which, by the use of a greater length of 
; discharge, became converted into two or more thick concentric 
ngs i 
the case with the dust figures of Lichtenberg. 
In my first paper, the term positive figure is applied to the one 
produced when the prime conductor charged with positive elec- 
tricity is made the positive electrode, and the sensitive plate the 
_hegative electrode; a mode of speaking which is common enough, 
‘@nd there traces its peculiar figure, and, correspondingly, that the 
negative electricity travels over to the sensitive plate (positive 
electrode), and there acts to generate a figure of a different kind. 
Ow the electric discharge. is rendered possible by the presence 
Of material partickes of some kind between the electrodes; in the 
Act of discharge these particles are heated till they become lu- 
‘Minous; if they consist of air, the discharge through them can 
be traced as a blue or violet line, while at or nearly at the same 
: * For No. 1, see volume xxxiii, page 219, 1862. : 
Ax. Jour. Scr.—Szconp Szriss, Vou. | No, 114—Noyv., 1864. 
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