on Permanent Magnetism. 513 



vanishes for a = 0, |3 = (as could be foreseen from the 

 properties of the irons a and /3, which have served as the 

 grounds of the argument), and is a maximum for a=/3. 



The number of the particles capable of remaining polarized 

 after the interruption of the magnetizing force will at most 

 form a quarter of the whole mass of the iron. 



If the magnetizing force be too feeble to produce saturation, 

 the lines OA, OrX will take up a position indicated ap- 

 proximately by the dotted lines. 



These considerations agree fairly well with many observed 

 facts. 



Thus all causes which tend to open, even temporarily or 

 only in certain directions, the network of the iron /3 (expan- 

 sion, shocks, vibrations) set at liberty some of the polarized 

 particles of a, and diminish the permanent magnetism. 



The greater the proportion of /3 iron the more difficult 

 becomes the polarization of a, in the diminished pores, and 

 the greater must the magnetizing force be to produce satura- 

 tion. 



When the proportion of carbon or of manganese is so 

 great that the whole of the iron on quenching or even without 

 this takes up the state /3, a substance will result which is 

 incapable of taking up magnetism ; for example Spiegel-iron 

 containing 25 per cent, of manganese and upwards, and Had- 

 field's steel. Since hardening takes place more energetically 



at the surface than in the interior of a bar, the relation -5- will 



P 

 always be smaller there. The permanent magnetism remain- 

 ing after saturation will therefore be stronger at the surface 

 if a > /3, which is the common case. It would be more feeble 

 if/3>a. 



Magnets formed of laminse are in general more powerful 

 than those of the same volume in a single piece, because 

 the laminae are more hardened. But the contrary should be 

 the case with metals in too hard a condition. 



It is difficult after a bar has been magnetized to saturation 

 in one direction to bring it to a moment of the same value in 

 an opposite direction, because certain particles of the a metal 

 being held immovably cannot be displaced, &c. 



The iron ft can also be looked upon as a medium whose vis- 

 cosity increases as its percentage in the mixture is greater. 



We can compare these ideas with the experiment of 

 Hughes, who has obtained permanent magnets by mixing 

 iron filings and wax and melting the latter, which in setting- 

 held fixed the polarized portions of iron. 



