Nipher — On the Nature of the Electric Discharge. 67 



nal, the electrical particles contained in the molecules 

 within the shadow, or between the tube and plate. The 

 positive column shown in Figs. A and C, Plate XXIV, 

 and all the figures of Plate XXV at any instant show to 

 the eye the same details which are shown in Fig. A, Plate 

 XXI. A time exposure in a camera shows, of course, 

 no such detail of the discharge. 



It appears that the insulated metal plate CC of Fig. 2 

 serves to separate the luminous positive column from the 

 Faraday dark space, to such an extent that they may 

 become apparent in discharges through air at ordinary 

 pressure. When the metal plate is removed, the mole- 

 cules at the boundary of these two regions mingle with 

 each other. Electrically they are friendly, and they are 

 being urged in opposite directions, by the compression 

 and rarefaction terminals of the machine. 



The Critical Spark-Length. 

 The Faraday Dark-Space. 



The metal plate CC of Fig. 2 was removed. A minute 

 spark-gap was made at a 1 . The contact at a was made 

 as complete as possible, so that no luminous point is seen 

 at this contact. The discharge then swept through the 

 entire spark gap of about 15 cm. The photograph of 

 this luminous column as taken by the camera is shown in 

 Fig. B, Plate XXV. The exposure was about five minutes. 

 The mica wind-mill shows a feeble wind from the positive 

 terminal. If the gap at a 1 is made somewhat larger, the 

 discharge is then filled throughout with small disruptive 

 sparks, and the windmill will not operate. If the gap a 1 

 is made still longer, as seen in Fig. D, Plate XXV, a 

 strong positive wind causes the windmill to rotate so 

 rapidly that its vanes are invisible. This wind sweeps 

 through the entire gap. The discharge is not then disrup- 

 tive in character. If the gap at a 1 is closed and that at a 

 is opened, the luminous streamers forming the positive 

 column are beaten back by a blast of air from the nega- 

 tive terminal. The mica windmill shows that the nega- 

 tive wind now sweeps the entire gap. Fig. C, Plate XXV, 



