Electricity 213 



306. There is a layer of coarse, fight-colored gravel over the tar 



on roofs, to keep the tar from melt ing. 



307. It is very easy to slip on a well-waxed hardwood floor. 



308. If you have a silver filling in one of your teeth and you touch 



the filling with a fork or spoon, you get a slight shock. 



309. You can shake a thing down into a bottle when it will not 



slip down by itself. 



310. If you rub a needle across one pole of a magnet three or four 



times in the same direction, then float it on a cork in water 

 one end of the needle will point north. 



SECTION 34. Conduction of electricity. 

 How does electricity travel ? 



Why do you get a shock if your hands are wet when you 

 touch a live wire? 



If you were to use a piece of string instead of a copper 

 wire to go from one pole of a battery to another or to 

 spin between the poles of the magnet of the dynamo, 

 you could get no flow of electricity to speak of. Electrons 

 do not flow through string easily, but they flow through 

 a copper wire very easily. Anything that carries, or 

 conducts, electricity well is called a good conductor. 

 Anything that carries it poorly is called a poor conductor. 

 Anything that allows practically no electricity to pass 

 through it is called an insulator. 



Experiment 6s. 1 Turn on an electric lamp. Turn it off 

 by opening the knife switch. Cover the blade of the knife 

 switch with a fold of paper and close it. Will the lamp 

 glow? Try a fold of dry cloth; a fold of the same cloth 

 wet. Connect the blade to the slot with a piece of iron ; 

 with a piece of glass; with porcelain; with rubber; with 

 dry wood; with wood that is soaking wet; with a nun. 

 Which of these are good conductors of electricity? Which 

 could be used as insulators? 



1 Read footnote, page 226, before doing this experiment. 



