PRACTICAL OPERATION OF DYNAMOS. 



135 shows two arresters like Fig. 134 connected to the two 

 mains of a 2,ooo-volt alternator. The Wurts arrester depends 

 solely upon the cooling effect of the massive brass cylinders for 

 extinguishing the arc across the spark gaps. 



Fig. 134. 



The "type K" arrester of the Westinghouse Company extin- 

 guishes the arc by smothering it. This arrester, now superseded 

 by the " MP" arrester of the same company, consists of a block 

 of hard wood to one flat face of which two metal plates are 

 attached. The spark gap is between the edges of these metal 



Choke coil 



to line 



= Spark gaps 



to earth 



to line 



2000 volt 

 alternator 



<park gaps 





Choke coil 



Fig. 135. 



plates, and the rather wide gap is bridged by the charred surface 

 of the wood. Another block of wood, with a small hole through 

 it for the escape of the hot vapors of the arc, is placed over the 

 spark gap as a cover, and serves to smother the arc. The charred 

 surface of the wood enables the lightning discharge to pass across 



