THE EUTECTOID TRANSITION POINT OF CARBON STEELS. 181 



The differences between the final ordinates seen in fig. 1 are due merely to 

 differences in the zero readings of the magnetometer in the different experiments, 

 the ordinates plotted being the actual scale-readings in each case. 



3. Comparison of Heating Curves. 



The heating curves for the alloys have one feature in common. With so many 

 curves superposed, in order to economise space, this feature is not perhaps brought 

 out with the maximum of clearness in fig. 1. But it can be seen that a drop in the 

 magnetism begins in all the steels at about the same temperature. Actually this tem- 

 perature is the same within one or two degrees. It is usually quite sharply marked. 

 For example, the thermocouple temperature was kept steady for several minutes at 

 733 C., in one case, without alteration in the magnetometer deflection which differed 

 very little from that at 730 C. The furnace temperature was then raised very slightly, 

 and, before the thermocouple registered 734 C., a rapid fall of magnetisation set in. 



This fall is absent or imperceptibly small in the iron curve and is smallest in that 

 for the steel weakest in carbon. Its relative magnitude increases with the percentage 

 of carbon, as fig. 1 shows, until the specimen containing (77 per cent, of carbon is 

 reached. After that, it is difficult to decide whether the ratio of the rapid fall 

 (below 740 C.) to the subsequent fall, before the magnetisation becomes too small to 

 be measurable, depends appreciably upon the percentage of carbon. 



The first important fact, then, is that the sudden loss of magnetism begins at the 

 same temperature in all the steels. This accords with the view that the eutectoid 

 patches, detected by the microscope, have always the same composition. It might 

 also suggest that this constant temperature is the true transition point between 

 eutectoid and homogeneous solid solution. 



That this inference would be wrong may be shown in two different ways. Fig. 2 

 shows one. The curves relate to successive interrupted heatings, of a steel containing 

 0'85 per cent, of carbon, described later.* At the moment it is only necessary to 

 call attention to the descending branches of the different curves. It will be noticed 

 that two (Nos. 4 and 5) proceed vertically downwards and one (No. 2) slopes slightly 

 outwards from left to right. The significant one, for the present purpose, slopes 

 inwards, i.e. towards the left. It (No. 3) was obtained with the most gradual rate 

 of heating and shows that the transition continues at a lower temperature than that 

 at which it began. 



This is a case of a phenomenon which is the converse of recalescence. In the 

 latter the material is self-warming. Here it is self-cooling. The effect is not very 

 pronounced and might, at first sight, be attributed to irregularities of heating ; but 

 the inference that it is due to lag can be justified in another way. This is shown 

 in fig. 3. 



* See 18. Fig. 2 is printed on p. 199. 



