738 



BEHAVIOUR OF PERMANENT MAGNETS. 



Magnets are liable to a gradual diminution of their 

 magnetism. To prevent this, keepers are kept in contact 



FIG. 371 (an. proj\, 



with the poles. When the magnets are in the form of 

 bars it is best to have a pair, arranged as in^ig. 372, 

 with opposite poles near each other, and connected on 

 each side by keepers of soft iron. 



Steel which has not been properly tempered, and 

 cast iron, stand with reference to their magnetic 



FIG. 372. 



behaviour midway between hardened steel and soft 

 wrought iron. They do not become magnetic by induc- 

 tion so easily as the latter, and they preserve their mag- 

 netism for some time, but not so permanently as highly 

 tempered steel. There exists also a native ore of iron, 

 called 'magnetic oxide of iron,' which may be rendered 

 permanently magnetic by a current or by magnetic in- 



