212 ELEMENTS OF ELECTRICAL ENGINEERING. 



of current entering the station over either line wire is dammed up 

 in front of the choke coil causing an electromotive force sufficiently 

 high to break across the spark gap G' or G" , thus establishing a 

 conducting path to earth. The function of the condenser may be 

 best explained in terms of a mechanical analogue as follows : A 

 wall is to be shielded from the excessive force of a hammer blow 

 by allowing the hammer to strike against a heavy ball of iron 

 which rests against the wall. The ball must move a little to take 

 up the momentum of the hammer. If the ball and wall were 

 both perfectly rigid the ball would fail to protect the wall. 

 Some slight degree of elasticity in the ball or wall is necessary, 

 or an elastic cushion must be interposed between the ball and 

 the wall. Each condenser shown in Fig. 128 acts like this elastic 

 cushion, permitting some movement through the choke coil of 

 current which does not necessarily flow on to and through the 

 dynamo, but merely charges the condenser. The condenser is 

 not often used with lightning arresters, inasmuch as the dynamo 

 and the connecting wires usually have sufficient electro-elasticity 

 or capacity. Lightning arresters installed out on a transmission 

 line do not require choke coils, but a choke coil should always 

 be inserted between the spark gap of a lightning arrester and the 

 generator or motor or other apparatus which is to be protected. 



The choke coil of a lightning arrester must be extremely well 

 insulated. The use of an iron core does not increase the induc- 

 tance of a coil with respect to excessively quick rushes of current, 

 inasmuch as the iron does not have time to become magnetized. 

 Lightning arrester choke coils are therefore usually made of a 

 single layer of coarse insulated wire wrapped on a porcelain core, 

 or of an insulated copper ribbon wound like a roll of tape. 



When a lightning discharge jumps across both gaps G' and 

 G", Fig. 128, the gaps constitute for an instant a low resistance 

 path between the line wires, and electric arcs, started by the light- 

 ning discharge, are maintained by current from the generator. 

 This virtually short-circuits the generator, hence provision must 

 be made for extinguishing the arc or arcs above described. The 



