CHAPTER XXVI 

 SIMPLE ELECTRICAL APPLIANCES AND MACHINES 



170. Electricity is the name given to an invisible agent 

 known to us only by the effects which it produces and by 

 various manifestations called electrical. These mani- 

 festations, a few centuries ago, were obscure and even 

 mysterious, but they are now comparatively well known 

 to the majority of people. However, the exact nature of 

 electricity is not yet sufficiently understood even by the 

 greatest scientists. 



171. Electrification by Friction. If a piece of hard 

 rubber or a stick of sealing wax is rubbed with flannel 

 or cat's fur and then brought near some dry bits of paper 



or pith balls, these light bodies will 

 jump toward the rod. As early as 

 600 B.C. the Greeks discovered that 

 rubbed amber had such characteris- 

 tics. Dr. William Gelbert, the father 

 ELECTRIFICATION BY o f mo dern science, in A.D. 1600 was 

 FRICTION , r ,. ^1^1 



the first to discover that these same 



The bits of paper . . . -. , , , , , , 



jump to and from the electrical effects could be produced by 

 glass rod which has a ru bbing together a great variety of 



positive charge. ? 



other substances besides amber and 

 silk; such, for example, as glass and silk, sealing wax 

 and flannel, hard rubber and cat's fur, or even by rub- 

 bing the hand on sheets of paper. 



The electrical charges produced on these objects by 

 friction have a peculiar relation much like the north- 



