Magnetism and Electricity 201 



leap and call it lightning. Much more leaps, however, 

 than leaped from the comb to your ear, and so it makes 

 a very much louder snap. The snap is caused in this 

 way: As the electric spark leaps through the air, it 

 leaves an empty space or vacuum immediately behind 

 it. The air from all sides rushes into the vacuum and 

 collides there ; then it bounces back. This again leaves 

 a partial vacuum ; so the air rushes in once more, com- 

 ing from all sides at once, and again bounces back. 

 This starts the air vibrations which we call sou ml. 

 Then the sound is echoed from cloud to cloud and from 

 the clouds to the earth and back again, and we call it 

 thunder. 



The electricity you have been reading about and 

 experimenting with in this section is called static elec- 

 tricity. " Static " means standing still. The electricity 

 you rubbed up to the surface of the comb or glass stayed 

 still until it jumped to the bit of paper or hair ; then it 

 stayed still on that. This was the only kind of electricity 

 most people knew anything about until the nineteenth 

 century; and it is not of any great use. Electricity 

 must be flowing through things to do work. That is 

 why people could not invent electric cars and electric 

 lights and telephones before they knew how to make 

 electricity flow steadily rather than just to stand still 

 on one thing until it jumped across to another and stood 

 there. In the next chapter we shall take up the ways 

 in which electrons are made to flow and to do work. 



Application 48. Explain why the stroking of a cat's back 

 will sometimes cause sparks and make the cat's hairs stand 

 apart ; why combing sometimes makes your hairs fly apart. 



