CHAPTER XVI 



GENERAL ACCOUNT OF MAGNETIC QUALITIES 

 OF SUBSTANCES OTHER THAN IRON / Jix ^ 

 AND STEEL 



Faraday's classification of all bodies as either paramagnetics or 

 diamagnetics General law Explanation of the action of the medium 

 by an extension of Archimedes' principle Ferromagnetics, para- 

 magnetics, and diamagnetics. 



Paramagnetics and diamagnetics. We have in the previous 

 chapters only described the magnetisation of iron and steel and 

 have mentioned that nickel and cobalt show sensible magnetisation 

 by induction, though in a far less degree than iron. They attract 

 and are attracted by the poles of a magnet and possess retentivity 

 in some conditions. Biot found that a certain needle of carefully 

 purified nickel which stands next to iron in its capacity for mag- 

 netisation retained its magnetism and possessed about one-third 

 the directive force towards the North of that of a certain steel needle 

 of exactly the same dimensions. Faraday experimented with 

 cobalt, which could easily be made to lift more than its own weight, 

 though losing all its magnetism on withdrawal of the inducing 

 magnet. 



Magnetisation was qfteii apparently detected by early experi- 

 menters in other metals and alloys, but we may probably ascribe 

 the effects they observed to the presence of small quantities 

 of iron. 



Towards the end of the eighteenth century it was observed 

 that antimony and bismuth were repelled from the pole of a strong 

 magnet. 



On the one hand, then, iron, nickel, and cobalt are attracted, 

 and on the other hand bismuth and antimony are repelled, by a 

 strong magnetic pole, and it was at one time supposed that other 

 substances possessed no magnetic properties at all. Faraday, how- 

 ever, showed that these two groups are really only at the two 

 extremes and that all bodies are either attracted or repelled, 

 though the forces are usually so slight that they can only be 

 detected with very powerful magnets. 



The mode of experiment by which he discovered this consisted 

 in suspending a small bar of the substance to be tested between 

 the poles of a strong electro-magnet by a fine thread. If 



