CHAPTER XXVIII 



MAGNETISM 



158. The Lodestone. Many centuries ago it was discovered 

 that a certain kind of iron ore had the curious property of 

 attracting other small pieces of iron. Ore of this kind is 

 found in many parts of the world, and is commonly known as 

 magnetite. If a piece of it be dipped into iron filings, they 

 will cling to it, and tacks, small nails, and other small objects 

 of iron or steel, may be picked up in this way. Pieces of this 

 ore were once called lodestones, or lead- 

 ing stones, but since the first were found 

 near Magnesia in Asia Minor, they have 

 finally come to be called magnetic stones 

 of natural magnets. A piece of steel, such 

 as a knife blade or needle, may be mag- 



FIG. 66. Magnets. (Tower, Smith and Turton.) 



netizedj or given magnetic properties, by stroking it with a 

 natural magnet, and it will then have all the properties of the 

 original magnet. Artificial magnets, made in other ways, are 

 now very common and are indispensable in electrical work. 

 In artificial magnets, the most common are bar magnets, 

 made of straight pieces of steel, and horseshoe magnets, whose 

 name is suggested by their shape. Steel, when magnetized, 

 becomes a permanent magnet, but soft iron can be magnetized 

 only temporarily. When a magnet is hammered, heated, or 

 twisted, it loses its magnetism. 



194 



