468 M. I. Pupin — Electrical discharges through poor 



Fl(jr9 



placed at the center of a 

 large glass bottle, as in- 

 dicated in the figure. 



A wide strip (c b) of 

 tinfoil was placed on the 

 outside, of the bottle, 

 covering about one-third 

 of the surface. The air 

 was exhausted through 

 the tube d, until the 

 pressure was about 3 mm . 

 The liquid in the bulb 

 a and the tinfoil were 

 connected to the secon- 

 dary poles of the induc- 

 tion coil. TV hen the 

 e m. f. was not too far 

 above the critical point 

 the discharge was in 

 form of numerous, quiv- 

 ering streamers, which 

 looked like the genera- 

 tors of a conical surface 

 with the center of bulb 

 a as vertex and the edge 

 of the tinfoil as directing curve. There was no visible dis- 

 charge between the bulb and the central parts of the tinfoil. 

 But the discharge spread out and gradually approached these 

 parts and at the same time the streamers became less numer- 

 ous and steadier, giving the discharge a more diffused appear- 

 ance as the potential gradually increased. When the e. m. f. 

 was gradually brought back to its original value the discharge 

 diminished in intensity but did not return to its original form 

 of distribution. It did that when the e. m. f. was consider- 

 ably lowered below its initial value, which showed that the 

 original distribution was not altogether due to the fact that at 

 any moment the density of the electrostatic charge of the tin- 

 foils was considerably larger near the edges. In this experi- 

 ment as well as in the preceding one the number of streamers, 

 their definition, their quivering motion, and their preference 

 for the paths along which the discharge started, increased with 

 the increase of pressure in the vacuum. A discharge (espe- 

 cially in vacua of poor conductivity) will always start between 

 parts of highest electrical density and each successive dis- 

 charge prefers the passage along the path of the first discharge 

 on account of the increased conductivity along this path. 



