. Nipher — On the Nature of the Electric Discharge. 65 



gaps may be made at a and a 1 , either by adjustment of 

 the positions of A and A 1 , or the floating terminals a 

 and a 1 . 



If the adjustment is such that loud sparks are passing 

 between the terminals, the effect of placing the insulated 

 metal sheet CC midway between the terminals is to com- 

 pletely prevent the sparks from passing. A luminous 

 brush " discharge" passes from the positive terminal to 

 the plate. A luminous glow is formed at the negative 

 terminal. This glow is of a character which indicates 

 that an active discharge is passing from the negative 

 terminal. But between this negative glow and the metal 

 sheet the space is absolutely dark. The positive lumi- 

 nescence and the negative glow are produced only when 

 a minute spark gap exists at a or a 1 . 



A photograph of these positive and negative discharge 

 effects is shown in Fig. A. Plate XXIV. This photograph 

 was made by means of a large copying camera, with an 

 exposure of 15 minutes. 



The distance between the knobs was about 13 cm. 



When the metal plate was hung on long silk cords it 

 set in stable equilibrium at a distance of 3 or 4 cm. from 

 the negative terminal. In this position sparks passed 

 through the plate as readily as they would pass when it 

 was removed. The dark space still existed between the 

 plate and the negative glow. If the plate is moved over 

 to the positive terminal the dark space follows, as the 

 positive column follows when the motion is in the oppo- 

 site direction. 



A small windmill having vanes of thin mica mounted 

 in a hub of hard rubber, and turning on pivots of vul- 

 canized fiber will revolve when placed in either gap. In 

 the positive column, the air is thus shown to be moving 

 towards the plate, and from the positive terminal. In 

 the dark space the air is moving in the opposite direc- 

 tion. In the positive column the rotation was about like 

 that which could be produced by walking with the wind 

 mill in still air with a velocity of 1.5 meters per second. 

 In the dark space the speed was somewhat less. 



