METEORIC AND ARTIFICIAL NICKEL-IRON ALLOYS. 
107 
increases ; but at the same time the IH curve becomes more and more clearly that of 
a mixture of magnetically hard and magnetically soft materials. 
§ 7. In the alloy containing 26'7 per cent. Ni the relative amount of the magnetically 
hard material, at —186° C., has so far increased that the initial fall of permeability 
during heating has practically disappeared. 
The decrease of permeability of the soft material during rise of temperature is 
almost exactly counterbalanced by the corresponding increase of that of the hard 
material. 
Thus over a considerable range of temperature we have a material of practically 
constant magnetic properties. We have, in fact, a magnetic analogue of the alloy of 
practically negligible thermal coefficient discovered by Guillaume. 
The 26'7 per cent. Ni alloy is a magnetic invar. Its BH curves are nearly identical 
for values of H between 25 and 400 over a temperature range of more than 300° C. 
(from —186° C. to at least +120° C.). 
§ 8. In the 24’4 and 24‘04 per cent. Ni alloys the permeability increases as the 
temperature rises from — 186° C., even in the strongest fields. A study of the data 
shows that the effect of a magnetically hard material now preponderates. The 
presence of the nickel-poor material is shown clearly, in the curves for the 24 per cent, 
alloy, by the marked increase of permeability (for H = 50) beyond 200° C., i.e., after 
the opposing influence of the nickel-rich material has disappeared. 
If the IH curves he plotted from the data given, it will be seen that the initial small 
amount of magnetism at the temperature of the air shown by the alloys containing 
less than 27 per cent, of nickel is that of a magnetically hard material and that, as 
the temperature falls, the hardness no longer increases rapidly with increase in the 
amount of magnetic material. 
§ 9. All the data above, together with others from the same paper which might be 
cited, admit of simple interpretation if it be assumed that alloys containing between 
24 and 30 per cent, of nickel consist mainly of a mixture of two materials, one nickel- 
rich and the other nickel-poor, forming a eutectic, and that the richer alloys contain 
a small excess of the nickel-rich material, whilst the poorer alloys contain a small 
excess of the nickel-poor material. 
From these data it appears that the artificial eutectic mixture is one in which the 
thermal variation of permeability is relatively small over a considerable range. 
A further important conclusion with respect to this eutectic is revealed by the 
behaviour after interrupted heating {cf. Section VI. above) of the 24‘4 per cent, alloy 
as described by Honda and Shimizu. 
If there were no thermal hysteresis, the eutectic, which appears at about 0° C. 
during cooling, should disappear at the same temperature during heating. That it 
does not do so seems due to thermal hysteresis in the nickel-poor constituent. The 
nickel-rich constituent apparently loses its magnetism, without appreciable hysteresis, 
at about the temperature corresponding to the percentage of Ni which it contains 
P 2 
