66 
MR. S. W. J. SMITH OH THE THERMOMAGNETIC ANALYSIS OF 
between these limits (see 106 and 107), lie only slightly below the curve, already 
determined, which represents the condition of the material when its permeability 
varies reversibly between the air temperature and a temperature not far below that 
of maximum permeability. (There is evidence of a slight diminution of permeability, 
during the course of the experiments, possibly due to oxidation or to some other 
cumulative effect.) 
Section Y.—The Relations between the Properties of Nickel-Iron 
Alloys Containing Different Percentages of Nickel. 
§ 1. A study of the relations between the temperatures of appearance and dis¬ 
appearance of magnetism in nickel-iron alloys containing different percentages of 
nickel {cf, e.g., Osmond, ‘Revue Generale des Sciences,’ vol. XIV., p. 865, 1903) 
shows that as the percentage of nickel increases up to some value not far from 27, 
the temperatures of appearance and disappearance become continuously lower and 
more widely separated from each other. 
When the percentage of nickel exceeds 30 the temperature of appearance is only 
slightly below the temperature of disappearance of magnetism, and Osmond is of 
opinion, in agreement with Guillaume, that the temperatures would practically 
coincide if the rate of change of temperature were made sufficiently slow to avoid 
possible difference between the temperature of the thermocouple and that of the bar 
of the alloy under examination. According to this view the alloys can be divided 
into two classes, namely, those containing not more than 27 per cent, of nickel, which 
are irreversible , and those containing more than 27 per cent., which are reversible. 
§ 2. These results suggest that the percentage at which transition occurs from one 
class of alloys to the other corresponds with a eutectic point. Following Roozeboom, 
Osmond represents the “equilibrium curves” for the nickel-iron alloys diagram- 
matically, as in the figure below {cf., however, § 19, p. 77). 
The curve ABCD is obtained from the magnetic measurements above described. 
The dotted curves AE, FCD are theoretical in the first instance, and their positions are 
not assumed to be more than qualitatively correct. Their properties may be indicated 
briefly by supposing a line drawn through any temperature 6, parallel to the axis 
of temperature, and cutting the different curves of- the figure in p, q, r, s, t , and u 
respectively. At this temperature 0 solid solutions containing 6q, dr, and 6u per 
cent, of nickel are, respectively, in equilibrium wdth “mixed crystals” of iron and 
nickel containing 6p, 6s, 6t per cent, of the latter. 
The curves are drawn in accordance with the theorem, confirmed by experiment on 
mixed substances which deposit mixed crystals, that when the transition temperature 
of a substance {e.g., Fe) is reduced by the addition of a second substance {e.g., Ni) 
with which it forms mixed crystals, the crystals in equilibrium with the solution 
must contain a less percentage of the second substance than the solid solution—the 
