ELECTRICITY. 69 



4. ELECTRICITY. 



Electricity generated by friction. When a glass tube is rubbed 

 with a piece of silk, it will be found to have acquired the property 

 of attracting light bodies, such as scraps of paper, sawdust, etc. 

 Moreover, if the tube be brought close to the face, a sensation similar 

 to that produced by the contact of a cobweb will be experienced. 

 If a knuckle be held near the tube, a peculiar noise is heard, and 

 a spark may be seen to pass between the tube and the knuckle. 



These phenomena show that the tube has acquired peculiar 

 properties by friction. It is said to be electrified, and the name 

 electricity is given to the cause producing these phenomena. The 

 term electricity is derived from the Greek electron, amber, in which 

 substance the property of attracting light objects after the applica- 

 tion of friction was first noticed about twenty-five hundred years ago. 



Conductors and non-conductors. Besides glass and amber, 

 there are many substances, such as sulphur, sealing-wax, hard 

 rubber, etc., which can be readily electrified by friction. On the 

 other hand, a bar of metal cannot be electrified unless it be fitted to 

 a glass rod, a piece of rubber, or to certain other substances, and 

 held by this handle while being rubbed with flannel. . Moreover, it 

 can be shown that a piece of glass or sulphur will attract particles 

 i. c., becomes electrified at that spot only where it has been rubbed, 

 while a tube of metal, fastened to a suitable handle, becomes elec- 

 trified over the whole surface of the tube. These facts show that 

 electricity when generated in such bodies as glass, sulphur, and 

 rubber, remains where it has been produced, while in metals it 

 immediately spreads over the whole mass. 



Bodies of the first kind, such as glass, etc., are said to be non-con- 

 ductors, while materials such as metals are said to be conductors. A 

 non 7 conductor is often called an insulator, and a conductor supported 

 by a non-conductor is said to be insulated. 



The reason that conductors, such as metals, cannot be electrified 

 by friction unless held by a non-conductor, is that the human body is 

 a good conductor, and therefore carries off the electricity as quickly 

 as it is generated. 



No substance is absolutely non-conducting, but the difference in 

 this power possessed by what are termed good conductors and non- 

 conductors is very great. Of conductors, may be mentioned : All 

 metals, charcoal, acids, saline solutions, living animals and vege- 



