242 ELEMENTARY LESSONS ON [CHAP. iv. 



depends upon the potential, and upon the pressure and 

 temperature of the air in which the discharge takes 

 place. The brilliance depends chiefly upon the quantity 



Fig. 107. x 



of electricity discharged. The colour of the spark varies 

 with the nature of the metal surfaces between which 

 the discharge takes place. Between copper or silver 

 terminals the spark takes a green tint, while between 

 iron knobs, it is of a reddish hue. Examination with 

 the spectroscope reveals the presence in the spark of the 

 rays characteristic of the incandescent vapours of the 

 several metals ; for the spark tears away in its passage 

 small portions of the metal surfaces, and volatilises 

 them. 



29O. Brush Discharge: Glow Discharge. If 

 an electric machine is vigorously worked, but no sparks 

 be drawn from its collector, a fine diverging brush oi 

 pale blue light can be seen (in a dark room) streaming 

 from the brass ball at the end of it farthest from the 

 collecting comb ; a hissing or crackling sound always 

 accompanies this kind of discharge. The brush dis- 

 charge consists of innumerable fine twig-like ramifications, 

 presenting a form of which Fig. 108 gives a fine example. 

 The brightness and size of the brush is increased by 

 holding a fiat plate of metal a little way from it. With 

 a smaller ball, or with a bluntly pointed wire, the brush 



