EXERCISES 



135 



by a passing thundercloud is greatest in projecting objects, such as 

 trees, spires, and hilltops (cf. 145). Commonly the leaves of trees 

 conduct the charge from the earth quietly, as a "silent discharge," 

 and prevent the striking of lightning. Franklin reasoned that a 

 pointed conductor a lightning rod would perform the same ser- 

 vice for a house. To be a real protection a lightning rod should have a 

 large diameter, should have a large number of points, and should 

 extend far enough 

 into the ground so 

 that it will always 

 end in moist earth. 



149. Exercises. 



1. When hair is 

 combed or brushed 

 vigorously in cold, 

 dry weather, it often 

 "fluffs out," showing 

 that the individual 

 hairs are repelling 

 one another. Ex- 

 plain. 



2. If, in cold, 

 dry weather, you 

 ' * skate " in your slip- 

 pers across a woolen 



carpet or rug, and then bring the tip of your finger near a gas jet or 

 other metal object, a spark often passes between your finger and the 

 object. Explain why. 



3. If you rub a stick of sealing wax with flannel, the wax receives a 

 negative ( ) charge. If the charged wax is brought near an un- 

 charged "electric pendulum," what happens? When the pendulum 

 is repelled, what kind of a charge does it have? If, now, you bring 

 near the charged pendulum a rubber comb which has been rubbed 

 with flannel, and you find that the comb repels the pendulum, what 

 kind of a charge is there on the comb? 



FIG. 124. 

 A Lightning Discharge. Field Museum of Natural History. 



