210 ELEMENTS OF ELECTRICAL ENGINEERING. 



point between the lamps connected to earth as shown in Fig. 1 20. 

 Whenever both line wires have grounds of the same resistance, 

 high or low, both lamps burn with equal brightness. Whenever 

 one line wire has a fairly low resistance ground, and the other a 

 high resistance ground, the lamps are unequal in brightness, and 

 the main which connects to the dim lamp is the main with the 

 low resistance ground. In order to distinguish between the un- 

 safe condition with both mains grounded through nearly equal 

 low resistances, and the safe condition with both mains grounded 

 through very high resistances, the lamps should be arranged to 

 be disconnected one at a time. If lamp A is dim when lamp B 

 is disconnected from main B, then main B has a high resistance 

 ground ; and if lamp A is bright when lamp B is disconnected 

 then main B has a low resistance ground. The reason for these 

 statements can be easily worked out by the reader by making a 

 diagram showing the relations of the two ground resistances to 

 the lamp or lamps. 



95. Lightning and lightning arresters. When a lightning stroke 

 occurs in the neighborhood of a transmission line, a sudden rush 

 of current takes place over the line due to one or more of the 

 following causes : (a) Electric charge accumulated on the line is 

 suddenly released and tends to flow to earth ; () the magnetic 

 action of the lightning discharge induces a sudden pulse of cur- 

 rent in the line ; (c) when the lightning actually strikes the line, 

 an enormous rush of current takes place as the total electric 

 charge of the lightning flash flows to earth. 



The damage to electrical apparatus by a very intense lightning 

 stroke may be a more or less complete destruction due to the 

 heat generated by the lightning discharge itself. Weak lightning 

 discharges are however very much more frequent than very 

 intense ones, and the damage done by a weak lightning discharge 

 is usually due to a puncturing of the insulation of the windings of 

 a machine and a consequent short-circuit in the machine which 

 causes it to be burned out by its own current. 



When the sudden rush of current which accompanies a light- 



