362 ELEMENTS OF ELECTRICAL ENGINEERING. 



iron wire retain a smaller portion of their magnetism, but with 

 greater persistence, and hardened steel retains a portion of its 

 magnetism very persistently, even when roughly handled. Mag- 

 netized bars of hardened steel are called permanent magnets. 



The more persistently a sample of iron or steel retains its resid- 

 ual magnetism, the greater the intensity of the magnetizing force 

 required to magnetize it. Thus hardened bars of steel are mag- 

 netized 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. 



On account of the tendency of iron to retain its magnetism, the 

 quantity 3 in equation (6) is not in general parallel to the magne- 

 tizing field dt t and the addition of 47rc7 and df is strictly a vec- 

 tor addition. However, the tendency of iron to retain its mag- 

 netism is usually neglected in practical magnetic calculations 

 relating to electromagnets. To neglect the tendency of iron to 

 retain its magnetism, is equivalent to assuming that <$, 3 and &C 

 are always parallel to each other. 



The one case in which the tendency of iron to retain its mag- 

 netism is considered in practical calculations of electromagnets is 

 in the calculation of energy losses in the iron. This calculation 

 is discussed in the articles on magnetic hysteresis. 



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

 by a magnetizing force of given intensity, the intensity of mag- 

 netization 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 this given value of eft is reached by a 

 decrease from a higher value, than if this given value of $C 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 3f y that is, to a state which is independent 



