CHAP, i.] ELECTRICITY AND MAGNETISM. 51 



they nearly touch with their blunt points. The disc 

 must rotate in the opposite direction to that in which 

 these tongues point. On the front side of the moving 

 disc and opposite the two armatures are two metal 

 combs, furnished with rows of points, and - joined 

 behind by brass rods, terminated with brass balls, m, 

 //, which, at first, must touch one another. ' To work 

 the machine, a small initial charge must be given by an 



Fig. 29. 



electrophorus, or by a rubbed glass rod, to one of the 

 two armatures. The disc is then rotated rapidly ; and 

 it is found that after a few turns the exertion required 

 to keep up the rotation increases greatly ; at the same 

 moment pale blue brushes of light are seen to issue from 

 the points, and if the rod m be drawn back so as to 

 separate the brass balls, a torrent of brilliant sparks 

 darts across the intervening space. The action of the 



