produced in Iron Wires by Magnetization, 347 



that I have seen, bearing on the result found by Joule. 

 Shelford Bidwell*, while investigating the subject, obtained 

 results the converse of Joule's, — indicating that not only 

 hardening but also annealing the iron diminished the elonga- 

 ting effect. He mentions one specimen that, when annealed, 

 contracted in length instead of elongating upon the applica- 

 tion of the feeblest magnetizing force. 



Barrett t in 1870 discovered that nickel, when magnetized, 

 contracts instead of elongating. Three years later A. M. 

 Mayer J published an account of his experiments on this 

 subject. His results, in the main, verified Joule's observa- 

 tions, with the exception of the action of hard steel. This 

 discrepancy was shown some years later by Bid well to follow 

 from their different methods of experimenting. Joule applied 

 a current of the same intensity but once, and both on making 

 and breaking the circuit observed an elongation ; while 

 Mayer used specimens already permanently magnetized, and 

 on observing the temporary magnetization found a contrac- 

 tion on making circuit and an elongation on breaking it. 



Mayer also observed hysteretic effects ; that is, the elonga- 

 tion due to a magnetizing force was less if the force had been 

 reached by successively increasing values than it was if the 

 current had been decreased from a maximum : the rod 

 remaining slightly elongated after the magnetizing force had 

 been removed : an effect analogous to the lagging of the 

 induction behind the magnetizing force. Nagaoka§ has dis- 

 cussed this phenomenon in the article cited, and has obtained 

 complicated curves showing the complete cycle of the hyste- 

 retic phenomena for both iron and nickel. 



The experiments mentioned were limited to comparatively 

 feeble fields. The work of finding the effects due to intense 

 fields has been most thoroughly done by Shelford Bidwell||, 

 who has found that rods do not continue to elongate indefi- 

 nitely with increasing strengths of field as the other investi- 

 gators supposed, but that a maximum value is after a time 

 reached. The rod then begins to shorten, and very intense 

 fields produce an absolute contraction which approaches a 

 limiting value asymptotically. He also experimented with 

 rings of iron, and with rods of steel, nickel, manganese steel, 

 cobalt, and bismuth. 



* Proc. Roy. Soc. vol. lv. p. 228. 

 + ' Nature,' 1882. 

 X Phil. Mag. [4] vol. xlvi. p. 179. 

 § Phil. Mag. '\p\ vol. xxxvii. p. 131. 



|| Proc. Roy. Soc. vol. xxxviii. p. 265 ; vol. xl. pp. 109 & 237 ; vol. xlvii. 

 p. 469; vol. lv. p. 228; Trans. Roy. Soc. vol. clxxix. (A), p. i05. 



