84 ELEMENTARY LESSONS ON [CHAP. H, 



98. Magnetisation by Currents of Electricity. 

 A strong current of electricity carried in a spiral wire 

 around a bar of iron or steel, magnetises it more power- 

 fully than in any of the preceding operations. In the 

 case of a soft iron bar, it is only a magnet, while the 

 current continues to flow. Such a combination is 

 termed an Electro-magnet ; it is fully described in 

 Lesson XXVI. Elias of Haarlem proposed to mag- 

 netise steel bars by passing them through a wire coiled 

 up into a ring of many turns, through which a strong 

 current was sent by a voltaic battery. Tommasi claims 

 to have magnetised steel bars by passing a current of 

 hot steam round them in a spiral tube : but the matter 

 needs further evidence. 



99. Destruction of Magnetism. A steel magnet 

 loses its magnetism partially or wholly if subjected to 

 rough usage, or if purposely hit or knocked about. It 

 also loses its magnetism, as Gilbert showed, on being 

 raised to a red-heat. 



100. Effects of Heat on Magnetisation. If a 

 permanent steel magnet be warmed by placing it in hot 

 or boiling water, its strength will be thereby lessened, 

 though it recovers partially on cooling Chilling a 

 magnet increases its strength. Cast iron ceases to 

 be attracted by a magnet at a bright red-heat, or at a 

 temperature of about 700 C. Cobalt retains its mag- 

 netism at the highest temperatures. Chromium ceases 

 to be magnetic at about 500 C, and Nickel at 350 

 C. Manganese exhibits magnetic attraction only when 

 cooled to 20 C. It has therefore been surmised that 

 other metals would also become magnetic if cooled to a 

 low enough temperature ; but a very severe cooling to 

 1 00 below zero destroys the magnetism of steel magnets. 

 The magnetic metals at high temperatures do not be- 

 come diamagnetic, but are still feebly magnetic. 



101. Forms of Magnets. Natural Magnets are 

 usually of irregular form, though they are sometimes 



