MAGNETISM. 407 



There are various methods of giving the mag- 

 netic property to steel or iron, but for all these a 

 magnet is necessary. 



If you merely take a bar of iron three or four 

 feet long, and hold it in a vertical position, you 

 will find that the bar is magnetic, and will act 

 upon another magnet ; the lower extremity of the 

 bar attracting the south pole, and repelling the 

 north pole. If you invert the bar, the polarity 

 will be instantly reversed ; the extremity which is 

 now lowest, will be found to be a north pole, and 

 the other extremity will be a south pole. 



This is easily explained, by supposing the earth 

 is itself to be a great magnet, and that the bar is 

 placed, by holding it nearly vertical, in the mag- 

 netical line, viz. in the direction of the dipping 

 needle. 



A bar of hard iron, or steel, will not answer for 

 the above experiment, the magnetism of the earth 

 not being sufficient to magnetize it. 



Bars of iron that have stood in a perpendicular 

 position are generally found to be magnetical, as 

 fire-irons, bars of windows, &c. 



Striking an iron bar with a hammer, or rubbing 

 it with a file, while held in this direction, likewise 

 renders it magnetical. An electric shock produces 

 the same effect ; and lightning often renders iron 

 magnetic. 



A magnet cannot communicate a degree of mag- 

 netism stronger than that which itself possesses ; 

 but two or more magnets joined together, may 

 communicate a greater power to a piece of steel, 

 than either of them possesses singly : hence we 

 have a method of constructing very powerful 

 magnets, by first constructing several weak mag- 

 nets, and then joining them together to form a 

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