Nipher—On the Nature of the Electric Discharge. 61 
the plates were exposed in open air in the darkened room 
containing the influence machine. No change in the re- 
sult was obtained when the table with its overhanging 
eurtains on which the plates were exposed, was wholly 
surrounded by a cage of galvanized wire screen which 
was then grounded. 
The lines of inflow on this series of plates are in ap- 
pearance like the outer extremities of the lines of inflow 
in Fig. B, Plate XVIII, and Fig. A, Plate XIX. 
When the exposure has reached the stage represented 
in Fig. A, Plate X XI, a spark is on the point of passing. 
In securing such a plate dozens of plates may be spoiled 
by the passage of a spark. Such a result is shown in 
Figs. B and C, Plate XXI. In the latter figure, the dis- 
ruptive discharge evidently did not begin at the grounded 
pin-head. That terminal was surrounded by a film of 
ionized air, which was thus sufficiently possessed of the 
property of conduction, to prevent the formation of the 
rarefied hole or channel through which the disruptive 
discharge passed. The end of this hole is about 4 mm. 
from the grounded pin-head. ‘The volley of negative 
electrons which passed through this discharge channel 
from the negative terminal, was apparently fired at the 
grounded pin-head across this small interval of ionized 
air. The result is seen in the fogging of that part of the 
plate around the grounded terminal. The volley was ap- 
parently a diverging one. Its fogging effect extended 
more than a centimeter beyond the grounded terminal at 
which the volley was directed. The pin-head protected 
that portion of the film which was behind it as seen from 
the muzzle of the discharge channel. Fig. A, Plate XXII, 
shows a somewhat larger region of ionized air into which 
the discharge from the air channel was diffused. In Fig. 
B two sparks passed, the first of which apparently pro- 
duced the ionizing effect. The second discharge passed 
through more than three centimeters of ionized air on its 
way to the grounded terminal and the fogging effect ex- 
tended nearly an equal distance beyond, as is shown by 
the shadow cast by the pin-head. 
