226 ELEMENTS OF GENERAL SCIENCE 



Artificial magnets, such as described above, are made by 

 rubbing steel with a loadstone (leading stone), a mineral 

 with a permanent magnetic quality, or by the use of an elec- 

 tric current. Magnetic tack hammers, telephone magnets, 

 magneto magnets, and toy horseshoe magnets are permanent 

 magnets; the magnet of an electric bell, however, is a tem- 

 porary magnet, the magnetism of which depends upon the 

 presence of an electric current in a coil of wire about the 

 iron. Permanent magnets gradually lose some of their mag- 

 netism, and may be damaged by contact with other magnets, 

 by heating, by mechanical shock (pounding, falling to the 

 floor, etc.), by failure to provide iron keepers for the ends, 

 and by abruptly placing the keeper in position. 



232. The region about a magnet. When a horseshoe mag- 

 net is dipped in iron filings or small nails (fig. 112), and 

 then removed, the metal usually forms a loop from one pole 

 to the other. When a glass plate is placed over the horseshoe 

 magnet, and fine iron filings are sprinkled evenly over the 

 plate, if it is tapped slightly the filings arrange themselves 

 in definite lines running from the north pole to the south 

 pole of the magnet. Each small piece of iron becomes a tiny 

 magnet and joins its neighbor, forming a connected line of 

 filings. The nails are affected in the same way, but since 

 they are larger, the line formed is irregular. 



These lines will always be found in some form near 

 magnets. For convenience they are called lines of force. In 

 some regions the lines will be numerous; in others only a 

 few will be found. Permanent maps of the lines of force 

 can be made by using blue-print paper or paper covered 

 with paraffin. In the latter case the paper must be warmed 

 slightly to " set " the filings. It will be found most inter- 

 esting to prepare such maps of the lines of force. Scientists, 

 engineers, and electricians find that the pictures of lines 

 of force assist in the understanding of motors, dynamos, 

 electromagnets, and electrical instruments. 



