1891.] Influence of Temperature upon Magnetisation. 109 



V. " On the Influence of Temperature upon the Magnetisation 

 of Iron and other Magnetic Substances." By HENRY 

 WILDE, F.R.S. Received May 8, 1891. 



In my paper on the " Unsymmetrical Distribution of Terrestrial 

 Magnetism,"* it was shown that by heating small surfaces of the thin 

 sheet iron entirely covering the ocean areas of the mapped globe 

 strong magnetic polarity was induced at the heated parts, just as 

 when the magnetic cdntinuity of the iron was interrupted by cutting 

 through the same parts of the iron in an equatorial direction. 

 Although this experiment appeared to me to demonstrate conclusively 

 that the magnetic power of iron was reduced by heating at compara- 

 tively low temperatures, and with small magnetising forces, yet, from 

 the contradictory results which have been obtained by other experi- 

 menters on the magnetisation of heated iron, directly opposite con- 

 clusions as to the magnetic intensities of the land and ocean areas 

 respectively might, with some reason, be drawn from those which I 

 had arrived at. 



Barlow, in an interesting paper on the magnetic behaviour of 

 heated iron,f refers to the discordant opinions which prevailed 

 on this subject among natural philosophers from the 17th century to 

 his time, and assigned the cause of these discordances to the obser- 

 vations being made with iron at different degrees of heat. 



Barlow found that the magnetic power of the bars of iron which he 

 experimented upon, as measured by the deflections of a compass needle, 

 increased with the temperature up to a dull red heat, at which it was 

 the strongest ; but, at a bright red heat, all magnetic action of the 

 iron suddenly disappeared. Scoresby,$ Christie, and others had also 

 noted a similar increase in the magnetic power of iron with increase 

 of temperature, when measured by the same means. 



Faraday, on the other hand, has described experiments to show 

 that the magnetic power of iron diminishes with increase of tempera- 

 ture. || He also found that iron at a bright red heat was not entirely 

 insensible to the action of large magnetising forces. 



More recently, Rowland,^" Baur,** and Hopkinson,ft by the em- 

 ployment of electro-dynamic methods, have also found an increase in 



* ' Koy. Soc. Proc.,' January 22, 1891. 



f ' Phil. Trans.,' 1822, p. 117, &c. 



I ' Edinburgh Koy. Soc. Trans.,' vol. 9, Part I. 



Christie on Effects of Temperature, &c., ' Phil. Trans.,' 1825, p. 62, &c. 



|| ' Phil. Mag.,' 1836, vol. 8, p. 177 ; ' Phil. Trans.,' 1846, p. 41. 



^ < Phil. Mag.,' 1874, vol. 48, p. 321. 

 ** ' Wiedemann, Annalen,' vol. 11, 1880, p. 403. 

 ft ' Phil. Trans.,' A, 1889, vol. 180, p. 443. 



