CHAPTER XXI 



LIGHTNING RODS 



Lightning flash. There are few persons of the present 

 day who are not familiar with electric currents and who 

 have not seen an electric spark or the flash which occurs 

 when an object is brought close to another which is charged 

 with electricity. The visible spark is not the electric cur- 

 rent, but white-hot air resulting from the resistance of the 

 air to the passage of the current. The presence of the cur- 

 rent is indicated by a snapping sound and by this light. 

 Scientists have proved that a lightning flash is due to the 

 heating of the air by the passage of an electric current 

 through it and that practically the same thing is taking place 

 between two clouds or between a cloud and the earth that 

 takes place between the poles of an electric machine when a 

 spark jumps from one to the other. The earth is charged 

 with negative electricity, and the clouds become charged 

 usually with positive electricity, but occasionally with 

 negative. When the accumulation becomes sufficiently 

 strong, a current will pass to another cloud or to the earth. 



Damage by lightning. The harm from a lightning flash 

 comes from the current itself and may cause death to ani- 

 mals, set fire to inflammable materials, or melt fusible 

 materials in exactly the same way that a strong current 

 from a dynamo will when an animal or object becomes 

 connected in the circuit. The cause of the generation of 

 electricity in the air has never been clearly or definitely 

 explained, but the effects indicate its presence, and, since 

 loss of life and destruction of property are caused by light- 

 ning, more or less attention has been given to methods of 

 protection from these natural electric currents. 



Greatest damage from lightning occurs in agricultural 



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