368 Mr. W. Sturgeon on the Theory of Magnetic Electricity. 



The retention of magnetic polarity is displayed to the 

 greatest extent by very hard steel. After this the retentive 

 faculty diminishes with various grades of hardness down to 

 soft steel ; thence by gradations downwards to the softest iron, 

 which exhibits the faculty of retaining magnetic polarity, only 

 in a very slight degree indeed. But the facility of magneti- 

 zing those bodies, and the extent to which their polarity is ex- 

 hibited, are in precisely the reverse order. 



Now, as the retention of polarity appears to result from a 

 want of facility, on the part of the metal, to readmit the mag- 

 netic matter which the exciting agent has arranged into active 

 polar lines on its surface and vicinal medium ; and as those 

 metals which display the retentive faculty in the greatest de- 

 gree also offer the greatest resistance to the formation of 

 those polar lines, or to the escape of the magnetic matter 

 from its ferruginous prison; — this disposition evinced by the 

 metal, of resisting both the egress and ingress of the magnetic 

 matter, must necessarily arise from a natural tendency which it 

 possesses to refuse the transmission of the magnetic element. 

 Hence those metals which retain magnetic polarity in the 

 highest degree may be called inferior magnetic conductors ; 

 and those which retain no traces of polarity after the exciting 

 process has ceased to operate, may be called superior mag- 

 netic conductors, with as much propriety, and for the same 

 reason, as similar terms are employed in electricity. 



Under these considerations it will appear that hard steel is 

 an exceedingly bad conductor of magnetism ; because it offers 

 a very great resistance to the motion of the magnetic matter. 

 This resistance causes the process of magnetizing to become 

 exceedingly tedious; and with very hard cast steel it very 

 seldom terminates successfully, or to the satisfaction of the 

 operator. Hence, in a practical point of view, it is interesting 

 to know that magnets constructed of cast steel should never 

 be harder than the blue temper. 



Soft iron being the best ferruginous conductor of mag- 

 netism, offers a much less resistance to the flow of the mag- 

 netic matter than when in any other state. The vigorous 

 polar magnetic lines are therefore speedily arranged, and to an 

 extent of concentration never to be accomplished on the sur- 

 face of very hard steel. 



But the same conducting quality which gives to soft iron a 

 facility of excitation, also gives a facility to the return of the 

 magnetic matter into the metal when the exciting agent is 

 withdrawn ; for which reason the retention of polarity dis- 

 played by soft iron is exceedingly feeble, and easily deranged. 

 Hence it appears that, as far as ferruginous bodies are con- 



