PRACTICAL OPERATION OF DYNAMOS. 



2I 9 



made from the line through a very short air gap to one of these 

 plates, and from the other plate to earth. A lightning discharge 

 passes through the block in the form of innumerable microscopic 

 sparks between the carbon grains, and no arc is developed. The 



Fig. 137. 



spark gap between the line and the first metal plate serves as a 

 complete break in the circuit from the line to ground, except of 

 course at the instant of a lightning discharge. This arrester is 

 suitable for both direct and alternating current circuits. 



96. Fire risks and the Underwriters' rules. Rules governing 

 the installation of electrical apparatus of all kinds have been 

 formulated by a National Conference on Standard Electrical 



