ELECTRICAL MACHINERY 



463 



Since all of the little particles of a charged substance, 

 because of their mutual repulsion, tend to get as far away 

 from each other as possible, the density of a charge is 

 very much greater on the ends of an oblong body than 

 in the middle. If the ends be drawn to a point the charge 

 will become so intense that the point cannot hold it all 

 and some of it will be given off to the air. 



610. Lightning and lightning rod. In 1752 Franklin 

 with his famous kite and key learned that there is elec- 

 tricity in the clouds. He also showed that lightning is 

 only a huge electric spark and that by means of points 

 like lightning rods these mammoth sparks may be dissi- 

 pated into the earth. As the cloud which is charged with 

 electricity approaches it induces an opposite charge in the 

 points and the charge is then quietly conducted away, 

 while if the points are not there the electric charge will 



assume such a volume that 

 when the cloud does give 

 it up it will strike the 

 building in such a great 

 bolt that damage is done. 

 From this it will be seen 

 FIG - 343 that lightning rods do not 



protect the building by conducting the whole charge of the 

 stroke away at once, but by diffusing and thus preventing 

 the charge collecting in large quantities. 



6 1 1. Potential difference (P.D.). If water is placed in 

 a tank A, Fig. 343, it will run through the f>ipe C into 

 tank B. We attribute the running of the water from 

 tank A to tank B to the difference in pressure between 

 the two tanks. In exactly the same way will a positive 

 charge of electricity flow from one body to another. 

 Thus, just as water tends to flow from higher pressure 

 to lower, does electricity of a higher potential flow to a 



