ELEMENTS OF ELECTRICITY AND MAGNETISM. 



force (a force which is repeatedly reversed in direction) produces 

 but little to and fro motion of a heavy body even though the 

 frictional opposition to motion be negligibly small. 



One of the most important uses of the choke coil is in connec- 

 tion with the lightning arrester. Figure 99 represents a dynamo 

 G supplying current to a trolley wire. When this wire is struck 

 by lightning a sudden rush of current takes place through G to 

 earth, and this rush of current may prove disastrous to the 

 dynamo by breaking through the insulation instead of following 

 the windings of wire in the machine. By placing a choke coil 

 C in the position shown in the figure, the lightning discharge is 



trolley wire 



B 



Fig. 99. 



ground 



made to break through a short air gap g and flow to earth 

 harmlessly. When the air gap g has been broken down in this 

 way, that is, when a spark or arc has been established across the 

 gap, it is a good conductor, and the dynamo G is short-circuited. 

 Therefore a lightning arrester must be provided with an arrange- 

 ment for stopping the flow of the dynamo current across the gap 

 g after the rush of current from the lightning stroke has ceased. 

 This is sometimes done, as in the Thomson arrester, by means 

 of a strong magnet which produces an intense magnetic field in 

 the region of the gap and pushes the arc sidewise, and blows it 

 out. This device is called the magnetic blow-out. The entire 

 arrangement of a choke coil 6", air gap g, and magnetic blow- 



