LIGHTNING. 223 



involved in the cloud is quickly discharged from one 

 particle to another, and accumulates on the surface of 

 the cloud ( 358). A cloud may thus become intensely 

 charged. The charge is generally positive. 



362, Lightning. When a cloud positively charged 

 floats over the earth, separated from it by a layer of 

 insulating air, the inductive influence of the cloud ren- 

 ders the ground beneath negatively electrified. Then the 

 cloud, ground, and insulating air, correspond respectively 

 to the inner and outer coatings and the insulating glass 

 of a Leyden jar. As the charge of a Leyden jar may be 

 made so intense that the mutual attraction of the sepa- 

 rated electricities will result in their rushing together and 

 thus piercing the jar ( 357), so the charge of a cloud 

 may become sufficiently intense to overcome the inter- 

 vening resistance and a lightning stroke ensues. Two 

 clouds charged with opposite electricities may float near 

 each other. Then they, with the intervening air, may be 

 looked upon as constituting a huge Leyden jar. Thus we 

 may see the lightning leaping from cloud to earth, or 

 from cloud to cloud. 



363. Lightning-Rods. The value of lightning- 

 rods depends upon the tendency of electricity to follow 

 the best conductor, and upon the effect of pointed con- 

 ductors upon electrical intensity ( 359). The lightning- 

 rod should, therefore, be made of a good conductor; 

 copper is better than iron. It should terminate above in 

 one or more points, tipped with some substance that may 

 be corroded or fused only with extreme difficulty. Plati- 

 num is a metal which satisfies these conditions very well. 

 The rod should extend above the highest point of the 



