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BELL SYSTEM TECHNICAL JOURNAL 



brought about by the change of pressure. More data are needed to 

 clarify the theory. 



There is an anomalous expansion of the high nickel alloys (due to 

 loss of magnetism) as the alloy is heated through the Curie point, 

 a contraction of the low nickel alloys, and no anomaly in the alloys 

 having about 70 per cent nickel, as indicated by the data ^^ of Fig. 16 

 on the expansion of these alloys in the range of temperatures including 

 the Curie points. Bethe's curve represents the change of interaction 

 energy with volume as a material is expanded or contracted, and it is 

 to be expected that there will be a reciprocal effect, a change in volume 



< 16 



O 

 *-> 10 



200 300 400 500 600 



TEMPERATURE IN DEGREES CENTIGRADE 



Fig. 16 — The expansion coefficient of some iron-nickel alloys, showing the magnetic 

 anomoly and its change in sign at about 70 percent nickel. 



as the material passes through the Curie temperature. More careful 

 consideration of the theory ^^ shows that the effect to be expected 

 does agree in sign with experiment. Also the disappearance of the 

 anomalous expansion occurs as expected at the same composition as 

 the maximum Curie temperature. 



Iron lies to the left of the maximum, as indicated by its expansion 

 curve. Calculations by Kornetski ^"^ indicate that the interaction 

 energy doubles for a 2 per cent increase in lattice constant. The 

 behavior of cobalt, nickel, and alloys of cobalt-nickel and of nickel- 



" M. Kornetzki, Z./. Physik, 98, 289-313 (1935). 



