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ELEMENTS OF ELECTRICAL ENGINEERING. 



the rather wide gap of this arrester without an excessive rise of 

 electromotive force. 



Fig. 136 shows a two-pole alternating current lightning 



Fig. 136. 



arrester of the General Electric Company. In this arrester the 

 arc is extinguished partly by the cooling effect of the massive 

 brass cylinders between which the spark gaps lie, and partly by 

 the effect of resistances connected in series with the spark gaps. 

 The middle one of the three short brass cylinders, which are 

 shown at the bottom in Fig. 136, is connected to ground, and the 

 brass cylinders at the sides are connected through the long 

 graphite resistance rods to the line wires. The double-pole 

 arrester shown in Fig. 136 is suitable for i,ooo-volt alternating 

 current mains. For higher voltages several spark gaps and 

 resistance rods are connected in series between each line wire and 

 the earth. 



Fig. 137 shows the parts of the "MP" lightning arrester of 

 the Westinghouse Company. This arrester, aside from the choke 

 coil which is of course separate, consists of a block of porous 

 material filled with small hard grains of carbon. This block 

 is clamped between two flat metal plates and connection is 



