ELECTRICITY. 71 



of the bodies following, but negatively electrified when rubbed with 

 one of those which precede it : 



1. Cat's skin. 5. Silk. 9. Sealing-wax. 



2. Flannel. 6. Human body. 10. Resin. 



3. Ivory. 7. Wool. 11. Sulphur. 



4. Glass. 8. Metals. 12. Gutta-percha. 



When an electrified body is brought to its normal condition it is 

 said to be discharged. The discharge may take place slowly through 

 the air, or rapidly by bringing the charged body in contact with the 

 earth directly or by means of a conductor, such as the human body 

 or a piece of wire. The discharge may be accompanied by a flash of 

 light, called a spark. 



Induction. A body charged with electricity will exert an influ- 

 ence upon surrounding unelectrified bodies and destroy the neutral 

 condition existing in them, attracting to the surface next to the elec- 

 trified body a charge the opposite to that which it contains. But 

 while one kind of electricity is drawn toward the original electrified 

 body, an equal quantity of the opposite kind of electricity is driven 

 toward the farther extremity of the bodies which are under the influ- 

 ence of the electrified body. This action exerted by an electrified 

 body on another body is called induction. 



Induction of bodies connected with the earth can last only so long 

 as they are under the influence of the electrified body, because as 

 soon as this is removed the induced electricity is immediately carried 

 off by the earth. But if an insulated body be brought near one that 

 is electrified, and while under its influence be touched at the end 

 opposite to the electrified body so as to carry off the electricity there 

 gathered, then on removing the electrified body the previously neu- 

 tral body will be found to be electrified with a charge opposite in 

 character to that of the originally electrified body. This method of 

 imparting electricity is called charging by induction. Induction 

 furnishes us the explanation of the attraction and repulsion of light 

 materials by electrified bodies. A positively excited body decdm- 

 poses the normal charge present in other bodies, and if sufficiently 

 light they move toward the excited body, discharging their negative 

 electricity and becoming charged positively through contact; they 

 are now repelled, as both bodies are electrified alike. 



Electrical machines. Electricity produced by friction or by 

 induction is called static electricity, or electricity at rest, to differ- 



