UNIVERSITY ) 



CHAP, ii.] ELECTRICITY AND MAGNETISM. 93 



iron filings by writing upon a steel plate with the wires 

 coming from a powerful voltaic battery. 



112. Surface Magnetisation. In many cases the 

 magnetism imparted to magnets is confined chiefly to 

 the outer layers of steel. If a steel magnet be put into 

 acid so that the outer layers are dissolved away, it is 

 found that it has lost its magnetism when only a thin 

 film has been thus removed. Magnets which have been 

 magnetised very thoroughly, however, exhibit some 

 magnetism in the interior. A hollow steel tube when 

 magnetised is nearly as strong a magnet as a solid rod 

 of the same size. If a bundle of steel plates are mag- 

 netised while bound together, it will be found that only 

 the outer ones are strongly magnetised. The inner ones 

 may even exhibit a reversed magnetisation. 



113. Mechanical effects of Magnetisation. 

 When a steel or iron bar is powerfully magnetised it 

 grows a little longer than before ; and, since its volume 

 is the same as before, it at the same time contracts in 

 thickness. Joule found an iron bar to increase by rjnnnnr 

 of its length when magnetised to its maximum. This 

 phenomenon is believed to be due to the magnetisation 

 of the individual particles, which, when magnetised, tend 

 to set themselves parallel to the length of the bar. This 

 supposition is confirmed by the observation of Page, that 

 at the moment when a bar is magnetised or demagnetised, 

 a faint metallic clink is heard in the bar. Sir W. Grove 

 showed that when a tube containing water rendered 

 muddy by stirring up in it finely divided magnetic oxide 

 of iron was magnetised, the liquid became clearer in the 

 direction of magnetisation, the particles apparently setting 

 themselves end-on, and allowing more light to pass be- 

 tween them. A twisted iron wire tends to untwist itself 

 when magnetised. A piece of iron, when powerfully mag- 

 netised and demagnetised in rapid succession, grows hot, 

 as if magnetisation were accompanied by internal friction. 



114. Action of Magnetism on Light. Faraday 



