O. N. Rood on the Electric Spark. 367 
formly black, or with the central portion only slightly lighter 
than the rest of the disc. It is true that uniformly shaded discs 
of this kind can be detected with the microscope on plates pre- 
pared with frictional electricity, but then they are much smaller 
than when the coil is used. Forms like V (in my first article on 
this subject) are also constantly produced. 
Form of the Negative Figure, 
viz: that which is generated on the sensitive plate used as the 
positive electrode. 
Length of discharge ;', inch. 
(a.) When the brass ball is used as negative electrode, a form 
like fig. 9 is produced, consisting of a number of dots, and gen- 
erally two circular discs with a central nucleus each. There are 
often many more dots than are indicated in the wood-cut. It will 
be seen that this figure closely corresponds with that produced 
by frictional electricity, with the exception of the two discs and 
nuclei, and these, as will be shown, are generated by the “Jumin- 
ous atmosphere.” : y ; 
(6.) When the negative electrode is a metallic point, we have 
a central nucleus, and a uniform disc of shade, fig. 10. This, 
with the exception of the central points, corresponds with what 
was often obtained in using frictional electricity. 
9. 10. 
c.) With two points, the figure remains as In 6. When a 
ae plate is aera many markings of a peculiar, but 
hardly starlike nature are revealed in the nucleus, see fig. 11. 
_It will be noticed that the size of the negative induction figure 
is nearly the same as that of the positive, whereas wl c- 
tional electricity, the negative figure was much Jarger. 
#; in. The forms are about the same as above indicated. 
3 in. (a.) When the negative electrode is the brass ball, the 
figure remains as above. 
(>.) When it is a point, a new fig- 
ure is produced, see figs. 12, which 
are magnified six diameters. These 
12. 
tricity. The central 
to the atmosphere. 
