50 
MR. S. W. J. SMITH ON THE THERMOMAGNETIC ANALYSIS OF 
fallen (from 37) to that of the air. It was found now to be even greater than in 
Experiment 1. It thus became clear that the relatively great value of the latter was 
not due to a time effect lasting over three months (as was at first thought possible), 
but to the fact that the material had been allowed to cool uninterruptedly from a 
temperature of about 550° C. (see Experiment n). If the permeability had been 
measured immediately after this cooling it would apparently have been found to be as 
high as it was three months later. 
An attempt was now made to find how the permeability of the ring in its new state 
altered with rising temperature. The result is shown in Experiments 40 to 44. These 
explained the apparently anomalous position of the points 3 and 4, which had been 
noticed at the time the latter were observed. The permeability varied slightly with 
rising temperature, falling at first and then rising. Between 300° and 400° C. it 
appeared to reach a maximum value, and then, beyond 400° C., it fell comparatively 
rapidly to a well-defined minimum value at about 435° C., beyond which the permeability 
quickly rose. It is obvious, therefore, from the data and the figure, that the points 
3 and 4 lay on a pd curve corresponding exactly with that just described. These 
experiments showed again that the behaviour of the material was definite, and, at the 
same time, that the permeability at a given temperature was strongly influenced by 
the previous thermal history of the material. 
The effect of interrupted heating having been so pronounced, I next examined 
whether any noticeable effects resulted from interrupted cooling. After the ring had 
been heated beyond the point at which the ferro-magnetism disappeared it was 
allowed to cool gradually until a temperature was reached at which the ferro¬ 
magnetism had begun to increase rapidly with fall of temperature. The permeability 
was measured at this temperature, and the ring was then heated through about 60° C. 
and kept at a constant temperature for about two hours, after which the permeability 
was again measured. The ring was then allowed to cool, and the permeability was 
measured a second time at the temperature at which the cooling was interrupted. 
The permeability rose considerably on heating, but on cooling it fell again practically 
to the same value as it had when the cooling was interrupted (see 45, 46 and 47). 
This result agreed with the qualitative diagram given by Guillaume as representing 
the behaviour of irreversible nickel steels under similar treatment (cf " C. E. Guillaume, 
‘ Congres International, Paris, 1900 ’). 
On cooling to the air temperature, the permeability was found to be a little lower 
than the lowest value previously recorded (see 48). Subsequent reheating beyond 
820 u C. and uninterrupted cooling restored the permeability at ordinary temperature 
to the normal value. Hence interrupted cooling had slightly lowered the subsequent 
ordinary temperature permeability (cf. § 7 below, p. 54, also Section VI., §11, p. 90). 
The object of the next experiments was to observe the effect of cooling interrupted 
at a temperature below that at which the permeability ceased to rise with falling 
temperature and began to diminish. The temperature chosen was about 465° C., at 
