PRACTICAL OPERATION OF DYNAMOS. 211 



ning discharge encounters a portion of the circuit which has 

 considerable electrical inertia or inductance, a very great electro- 

 motive force is created between the terminals of that portion of 

 the circuit, in the same way that an enormous mechanical force is 

 created when a moving body strikes a heavy stationary body. 

 Thus a sudden rush of current coming into a station over a trans- 

 mission line, encounters the highly inductive windings of wire on 

 the field or armature of a dynamo. The electrical inertia (induc- 

 tance) of the winding dams up the rush of current, as it were, and 

 the current rush is almost sure to break through the insulation 

 at the very entrance to the winding, passing from the copper wire 

 to any metal that is connected more or less thoroughly to earth. 



The lightning arrester is a device for shielding a dynamo or 

 other electrical machine from the rushes of current which come 

 into a station on an overhead line during a thunderstorm. 



Fig. 128 is a diagram showing the essential features of a light- 

 ning arrester, or rather, 



to line >ri/ ,_, to dynamo of tWO distinct light- 



T ^ \\\\C f 



ning arresters, one for 



to earth , L^ each line wire. Such a 



pair of lightning arrest- 



*c" X" \\\\C" ers wnen mounted on 



to line T /oooorx ^ to dynamo , , n i 



' r OOOO^ - 1 - one base plate is called 



Fig> 128 ' a double-pole arrester. 



Each of the line wires upon entering the station building is 

 connected to a coil of wire, called a choke coil or reactance 

 coil, whence the wire leads on to the dynamo or other machine. 

 Just in front of the choke coils each line wire has a ground 

 connection broken by a short spark gap G, and just behind the 

 choke coils each line wire should have a ground connection 

 through a condenser C with very thick insulation. Fig. 1 28 shows 

 every essential feature of a lightning arrester except the device 

 for extinguishing the arc which tends to persist at the spark gaps 

 G f and G" after the lightning discharge has passed to earth. The 

 action of the apparatus shown in Fig. 128 is as follows : A rush 



