1894.] Dimensions of Wires and Eings of Annealed Iron. 229 



reached as high a value as 540 units, beyond which point the experi- 

 ment was not carried. 



On reference to the original paper in the ' Comptes Rendus,'* it 

 appeared that the experiment was made with a cylindrical bar, 

 measuring 5'2 cm. by 1*95 cm., its length being less than three times 

 its diameter. The actual magnetising force must therefore have been 

 enormously less than that due to the coil itself, and it is very improb- 

 able that it ever reached the value at which the iron (or the greater 

 portion of it, for the magnetisation must have been far from uniform) 

 would attain its maximum extension. 



The ' Phil. Mag.' of December, 1893, contains an account of some 

 experiments by Mr. Sidney Lochner, made, as he states, with the 

 express object of determining whether M. Berget's results or my own 

 were correct. Using a thin iron rod, he obtained a curve very closely 

 resembling those published by myself ; but, as might have been 

 expected, he found that a short thick rod, like that of M. Berget, 

 gave no decided indication of having passed a maximum extension 

 within the limits of the magnetising force that he employed. 



A paper of great interest, on " Hysteresis attending the Change of 

 Length by Magnetisation in Nickel and Iron," by Mr. H. Nagaoka, 

 published in the ' Phil. Mag.' for last January, also incidentally con- 

 firms my observations. 



In subsequent papers I have shown how the elongations and retrac- 

 tions were modified by tension, and by the passage of electric currents 

 through the specimens under examination ; and in the present com- 

 munication I propose to deal with the somewhat unexpected effects 

 which were produced by carefully annealing the iron before using it. 



Upon this point, Joule's observations are as follows : " On 

 inspecting the tables, it will be remarked that the elongation is, for 

 the same magnetic intensity, greater in proportion to the softness of 

 the metal. It is greatest of all in the well annealed iron bars, and 

 least in the hardened." f 



The current belief, in which till quite recently I myself shared, is 

 in accord with this statement. It appears, however, as will be shown, 

 that it represents the reverse of the truth. Joule's conclusion with 

 regard to iron seems to have been based entirely upon the results of 

 a single experiment with an unannealed bar, and he may possibly 

 have been mistaken in supposing that it consisted of the same quality 

 of iron as the annealed bars which he had used before. 



My own experiments, the results of which are given in Table I and 

 fig. 1, were made with a piece of iron wire, 1O6 cm. in effective length 

 and 0*265 cm. in diameter. The curve marked (1) shows the be- 

 haviour of the iron in the condition in which it was bought. Its 

 * 'Comptes Rendus,' November 7, 1892, p. 722. 

 t Joule's ' Works/ p. 246. 



