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 

 curtains 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 XXI, 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. 



