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Proceedings of the Royal Society of Edinburgh. [Sess. 
Diagram III. is of considerable interest. In obtaining the curves there 
shown the following procedure was gone through : — The specimen was placed 
within the solenoid and a magnetising field applied. This field was 
maintained constant throughout. The temperature of the specimen was 
now gradually increased until the magnetometer deflection had become zero ; 
the specimen was then allowed to cool slowly to room temperature, readings 
of the magnetometer being taken as the temperature fell. Such curves 
were first obtained for the Heusler alloy by Mr Y. Bruce Hill, and are 
described by him in the Physical Review, vol. xxi. , p. 335. 
The diagram shows the curves obtained with the specimen subjected to 
applied fields of amounts 150 c.g.s. units, 75 c.g.s. units, and 05 c.g.s. units 
respectively. It will be seen that in the case of the higher fields the 
permeability of the material is zero at about 500° C., and that the cooling 
curve in no case coincides with the heating curve. At the higher fields 
also, as cooling proceeded the susceptibility remained practically zero, until 
the temperature had fallen to about 350° C. The contrast between the 
behaviour of the material when heating and when cooling is even more 
striking when the applied field is weak, as will be seen from the diagram. 
Hopkinson* has carried out similar experiments upon alloys composed of 
nickel and iron, and it is interesting to contrast the results obtained with 
those exhibited in Diagram III. In Hopkinson’s tests the specimens were 
in the form of rings, and the ballistic method of examining the magnetic 
quality was employed, the temperature being inferred from the resistance 
of the secondary coil. A sample containing 4 - 7 per cent, of nickel and 022 
per cent, of carbon exhibited magnetic properties, when tested at room 
temperature, very similar to those of mild steel. On gradually increasing 
the temperature it was found that the magnetic quality of the material 
gradually improved until a temperature of about 750° C. was reached, after 
which it deteriorated rapidly, and the susceptibility became zero at a 
temperature of about 825° C. On cooling, the susceptibility remained zero 
until a temperature of about 670° C. had been reached, after which point 
it increased, until finally at room temperature it was restored to very nearly 
its initial value. 
The behaviour of a nickel-iron containing 25 per cent, of nickel was even 
more remarkable. This specimen was found to be non-magnetic at room 
temperature, but on being cooled to about — 50° C. it became magnetisable, 
and remained so as the temperature was gradually increased to about 600° C., 
at which point the susceptibility became zero. On allowing the specimen 
* Hopkinson, Proceedings of the Royal Society., December 12, 1889 ; January 23, 1890 ; 
May 1, 1890. 
