8 ON MAGNETISM. 



general, suffer no deterioration from this employment. 

 The new steel magnet will retain its magnetism through 

 an indefinitely long period : its permanency depending 

 greatly on the quality of the steel. The steel best 

 adapted for large magnets is that which is best for fine 

 cutlery : and the steel should be perfectly hard through 

 the whole length of the bar. For compass-needles, the 

 same steel at spring-temper is found more advan- 

 tageous. 



It is possible, by various contrivances, as for instance 

 by holding both the hand-magnets with the red ends 

 downwards or both with the blue ends downwards, to 

 create a magnet with a concentration of magnetism in 

 the middle of its length. Such magnets however are 

 useless, and we shall not further notice them. 



Some artists prefer the following method of magnet- 

 izing simultaneously three or more bars. The bars are 

 laid so as to form a closed circuit, and a powerful horse- 

 shoe magnet is placed upon any one bar with both its 

 ends in contact with the bar, and is carefully carried 

 thus round the whole circuit of bars, always in contact, 

 and with the same end of the horse-shoe magnet always 

 preceding. (See Figure 2.) The circuit is repeated 



Fig. 2. 



