ON THE MAGNETISATION OF IKON. 33 



Report of the Committee, consisting of Professor Fitzgerald 

 (Chairman), Professor Barrett (Secretary), and Mr. Trouton, 

 appointed to investigate the Molecular Phenomena connected 

 luith the Magnetisation of Iron. 



(DwiNG to various causes, it is not proposed on the present occasion to do 

 more than present a formal interim report, reserving to next year the 

 full report of the Committee. 



So much work has of late been done on the general subject of the 

 molecular phenomena attending magnetisation, that it would now be 

 beyond the scope of the Committee adequately to report on the whole 

 matter. We propose, therefore, . in the first instance, to confine our 

 report to those phenomena accompanying the so-called critical tempera- 

 ture of iron — that point in or near which the magnetic state is lost in 

 heating and regained on cooling. Here, at a dull red heat a series of 

 profound and remarkable changes occur in iron and steel, to which atten- 

 tion was called in a paper ' On certain remarkable Molecular Changes 

 occuri'ing in Iron Wire at a Low Red Heat,' read by the Secretary of 

 this Committee at the meeting of the British Association at Bradford 

 in 1873, and subsequently published in the ' Philosophical Magazine ' for 

 December 1873. In this paper the phenomenon known as the rerxilescence of 

 iron was first published, it having been discovered by the author early in 

 September 1873. The principal points in that paper were as follows : — 



(1) Mr. Gore, in 1869, had discovered that a momentary elongation of 

 iron occurred in cooling after heating a wire of that metal to a white 

 heat. In 1873 the author fonnd a similar but reverse action took place 

 in heating the wire. (2) This anomalous deportment was found, both in 

 heating and cooling, approximately to coincide with, on the one hand, 

 the Zoss, and on the other with the resumption of the magnetic state of 

 iron or steel. (3) At the critical temperature the wire, having cooled 

 down to a dull red heat, suddenly flashed into a bright glow ; likewise, 

 during the heating of the wire the temperature remains stationary for a 

 short time when the critical temperature is reached ; a rise in the specific 

 heat of iron and steel therefore occurs at the critical temperature. (4) 

 A curious crepitating sound occurs at the critical temperature, similar to 

 that heard in the magnetisation of iron, or in the production of the scales 

 of oxide on the wii-e. (5) Professor Tait's remarkable thermo-electric- 

 change ia iron occurs at this same temperature. (6) Hard iron wire and 

 steel wire exhibit recalescence, but certain specimens of good soft iron 

 failed to show it, and even \n the wire that exhibits it the phenomenon 

 grows less marked after repeated heating and cooling. 



To these observations may be added that a recent investigation by the 

 Secretary of the Committee on the properties of 14- per cent, manganese 

 steel wire (' Proceedings,' Royal Dublin Society, December 1886) shows, 

 that this body, which is almost a non-magnetic metal, does not exhibit 

 the anomalous deportment observed in ordinary steel wire. This fact i& 

 of considerable interest, as linking the foregoing phenomena more 

 closely with the magnetic state of iron and steel. 



Estimating the temperature of the critical point approximately from 

 the expansion of platinum brought to the same degree of redness, it was 



1889. D 



