CHAP, iv.] ELECTRICITY AND MAGNETISM. 243 



appears smaller, but is more distinct and continuous. 

 The brush discharge is larger and more ramified when a 

 positive charge is escaping, than when the electrification 



Fig. 108. 



is negative. Wheatstone found by using his rotating 

 mirror that the brush discharge is really a series of 

 successive partial sparks at rapid intervals. 



If the blunt or rounded conductor be replaced by a 

 pointed one, the brush disappears and gives place to a 

 quiet and continuous glow where the electrified particles 

 of air are streaming away at the point. If these con- 

 vection-streams are impeded the glow may once more 

 give place to the brush. Where a negative charge is 

 being discharged at a point, the glow often appears to 

 be separated from the surface of the conductor by a dark 

 space, where the air, without becoming luminous, still 

 conveys the electricity. This phenomenon, to which 

 Faraday gave the name of the " dark " discharge, is very 

 well seen when electricity is discharged through rarefied 

 air and other gases in vacuum tubes. 



291. Length of Sparks. Roughly speaking, the 



