178 DR. S. W. J. SMITH AND ME. J. GUILD: A THERMOMAGNETIC STUDY OF 



is excess of iron. During heating the reverse changes take place. The eutectoid 

 transforms first and then the iron or the carbide as the case may be. 



Qualitative evidence of these statements is provided by the microscope, and the 

 theory which accounts for them gives rise to the so-called equilibrium diagram based 

 upon thermal observations during cooling. It occurred to us that the thermomagnetic 

 method might throw some useful light upon that part of the equilibrium diagram 

 which relates to the appearance and disappearance of the eutectoid. This opinion 

 was based upon the fact that, for reasons which need not be dwelt upon, when the 

 eutectoid or any portion of it disappears during heating the iron in it should lose 

 practically all its magnetism. Conversely, when the reverse change takes place during 

 cooling, that magnetism should be regained. 



2. Materials Used and Methods of Measurement. 



Most of the specimens of steel examined were cut from materials, described in 

 earlier papers,* which were given to us by Mr. E. A. WRAIGHT and by Prof. J. 0. 

 AENOLD, F.R.S., respectively. The analyses supplied, showing percentages of 

 elements other than iron, were as below : - 



Traces of sulphur and of phosphorus (not exceeding 0'03 per cent, in each case) 

 were also present. As in the paper last cited, the specimens were cut in the form of 

 tubes, each 7 cm. long, of which the external diameter was 5 '5 mm. and the internal 

 3 mm. 



The temperature was measured by means of a platinum-rhodium platinum thermo- 

 couple, calibrated by means of a platinum thermometer, of which the junction was 

 placed near the centre of the tube under examination. This tube was contained 

 within a copper tube, about 11 cm. long and 1 mm. thick, of very slightly more than 

 5 '5 mm. internal diameter, from which it was separated by very thin strips of mica. 



The platinum heating coil was wound bifilarly upon a layer of asbestos paper 

 wrapped round the copper tube. Several thicknesses of asbestos paper were then 

 wrapped over the heating coil, and the resulting cylinder, about 15 cm. long, was 



* ' Proc. Phys. Soc.,' XXIV., pp. 62-69, and pp. 342-349, 1911. 



