ATMOSPHERIC ELECTRICITY. 



face. Similar experiments on an elongated cylinder, like 

 the prime conductor of the electric machine, show that 

 the density is greater at the ends. On an ovoid conductor, 

 like that shown in Fig. 160, the density is 

 greatest at the smaller end. In general, the 

 electric density is greatest on those parts of 

 a charged conductor which project the most 

 and have the sharpest ends. This tension 

 at a point may become so great that the 

 FIG. 160. electricity will escape rapidly and quietly. 

 This explains the action of points, which plays so impor- 

 tant a part in the action of electric machines. This 

 property will he illustrated in several of the experiments 

 in 371. It is also fundamental to the action of lightning- 

 rods. 



360. Atmospheric Electricity. The identity of 

 lightning with electricity, though long suspected, was first 

 proved by Franklin's famous kite experiment. The at- 

 mosphere is, at all times, more or less electrified. The 

 kind and intensity of this atmospheric electricity varies at 

 different times. In fair weather, the atmospheric elec- 

 tricity is generally positive. The friction of moving masses 

 of air probably contributes to the presence of atmospheric 

 electricity. 



361. Electrified Clouds. Dry air being a poor 

 electric conductor ( 333), the air particles discharge 

 their electricity into each other slowly and with difficulty. 

 The electricity thus prevented from accumulating has 

 little density or tension, and hence gives few manifestations 

 of its presence. The moist particles which constitute a 

 cloud being good conductors, the atmospheric electricity 



