214 ELEMENTS OF ELECTRICAL ENGINEERING. 



conducts a current rush nearly as well as a solid rod of graphite, 

 inasmuch as a very sudden rush of current flows only through 

 the surface layers, or skin, of a conductor ; but it offers an enor- 

 mously greater resistance 

 than a solid graphite rod to 

 a current which is steady or 

 which changes slowly* in 

 value. Therefore a rod of 

 clay coated with graphite of- 

 fers an easy path for a light- 

 ning discharge, but prevents 

 the flow of the generator cur- 

 rent which tends to maintain 

 the arc. 



Fig. 129 shows a single- 

 pole magnetic blow-out di- 

 rect current lightning ar- 

 rester manufactured by the 

 General Electric Company. 



This particular arrester has two spark gaps in series. The 

 main spark gap is between the two wing-shaped pieces of brass, 

 and it lies between the poles of an electromagnet. The auxiliary 

 spark gap is between the two rounded brass points shown at the 

 right in Fig. 129. The winding of the electromagnet forms a 

 shunt around the auxiliary spark gap, and the current which con- 

 tinues to flow across the two gaps of the arrester after a lightning 

 discharge has passed, quickly establishes itself in the magnet 

 winding, excites the magnet, and blows out the arc in the main 

 gap. 



Figs. 130, 131 and 132 show a compact form of magnetic 

 blow-out lightning arrester enclosed in a porcelain receptacle. 



* The current rush which constitutes a lightning discharge sometimes grows at a 

 rate exceeding $00,000,000 amperes per second during the few millionths of a second 

 that it continues to grow. Compared with this the maximum rate of growth of an 

 ordinary commercial alternating current, namely, a few thousand amperes per second, 

 is very slow indeed. 



