88 ADVANCED ELECTRICITY AND MAGNETISM. 



therefore a short thick bar does not retain as much residual 

 magnetism as a long slim bar, or as a bar which forms a closed 

 or nearly closed magnetic circuit. 



A long slim bar or a closed ring of soft annealed wrought iron 

 or mild steel may retain as much as 90 per cent of its initial 

 magnetism. But a very weak demagnetizing field or a slight 

 mechanical shock is sufficient to cause the soft iron or steel to 

 lose nearly all of its residual magnetism; cast iron and hard- 

 drawn iron wire retain a smaller portion of their magnetism, 

 but with greater persistence; and hardened steel retains a por- 

 tion of its magnetism very persistently, even when roughly 

 handled. Magnetized bars of hardened steel are called permanent 

 magnets. 



The more persistently a sample of iron or steel retains its 

 residual magnetism, the greater the intensity of the magnetizing 

 force required to magnetize it. Thus hardened bars of steel are 

 magnetized by placing them between the poles of a strong electro- 

 magnet or by placing them in a coil of wire through which a strong 

 electric current is caused to flow. 



56. Magnetization curves. When an iron rod is acted upon 

 by a given magnetizing force, the intensity of magnetization of 

 the rod (and also the flux density in the rod) may have any value 

 whatever (between certain limits) , on account of the tendency of 

 the iron to retain its previous magnetic state. Thus the intensity 

 of magnetization of an iron rod is much greater for a given value 

 of &C, if the given value of $ is reached by a decrease from a 

 higher value, than if the given value of <$' is reached by an 

 increase from a lower value. 



If, however, an iron rod is subjected to mechanical shocks^ 

 or if the magnetizing field pulsates slightly before it settles to a 

 steady value, then the rod tends to settle to a state which depends 

 only on the value of &C, that is, to a state which is independent 

 of the previous magnetic condition of the rod. Under these 

 conditions, therefore, definite values of & and 3 are produced 



