374 WELLS'S NATURAL PHILOSOPHY. 



facilitate its passage arc called conductors ; those tliat re- 

 tard, or almost prevent it, are called non-conductors. 



No substance can entirely prevent the passage of electricity, nor is there 

 any which does not oppose some resistance to its passage. 



"What Eiib- 0^' ^^^ bodies, the metals are the most per- 

 comhictort^^'of ^^ct couductors of clcctricity ; charcoal, the 

 electricity? earth. Water, moist air, most liquids, except 

 oils, and the human body, are also good conductors of 

 electricity. 



What IB the *^^^- The velocity with which electricity 

 tricuyr"^ *''*"'' passes through good conductors is so great, 

 that the most rapid motion produced by art 

 appears to be actual rest when compared to it. Some 

 authorities have estimated that electricity will pass 

 through copper wire at the rate of two hundred and 

 eighty-eight thousand miles in a second of time — a ve- 

 locity greater than that of light. The results obtained, 

 however, by the United States Coast Survey, with iron 

 wire, show a velocity of from 15,000 to 20,000 miles per 

 second. 



What snh- Gum sliellac and gutta percha are the most 



conTuct^rsTf' P^rfcct nou-conductors of electricity ; sulphur, 

 electricity? sealing- wax, resin, and all resinous bodies, 

 glass, silk, feathers, hair, dry wool, dry air, and baked 

 wood are also non-conductors. 



Electricity always passes by preference over the best 

 conductors. 



Thus, if a metallic chain or -wire is held in the hand, one end touching the 

 ground and the other brought into contact with an electrLfied body, no part 

 of the electricity will pass into the hand, the chain being a better conductor 

 than the flesh of the hand. But if; while one end of the chain is in contact 

 with the conductor, the other be separated from the ground, then the electricity 

 will pass into the hand, and vnll be rendered sensible by a convulsive shock. 



WTienisabody 7^44. Whcu 0. couductor of electricity is sur- 

 rounded on all sides by non-conducting sub- 

 stances, it is said to be insulated; and the non-conducting 

 substances which surround it are called insulators. 



