808 riiOCEEDIiNGS OF THE AMERICAN ACADEMY 



VIII. 



BRIEF CONTRIBUTIONS FROM THE PHYSICAL LABORATORY 



OF HARVARD COLLEGE. 



No. I. 



ON THE EFFECT OF HEAT UPON THE MAGNETIC SUS- 

 CEPTIBILITY OF SOFT IRON. 



Br H. Amort and F. Minot. 

 Presented, Jan. 12, 1875. 



The determination of the question whether heat influences the capa- 

 bilities of soft iron to be magnetized appears to us to be an interesting 

 question, since, in the later forms of magneto-electric engines, the 

 armatures necessarily become heated by their movement in a magnetic 

 field. The question is also of interest from a molecular point of view. 

 We have confined ourselves to the determination of the effect of such 

 heating upon the induced currents produced by suddenly passing an 

 electric current about the bar of soft iron, which is heated to diflerent 

 temperatures. 



Great dithculty was anticipated at first in determining the tempera- 

 tures of the bar at different times. Preliminary experiments show, 

 however, that the question resolved itself into observing the decided 

 changes at the temperature of dull red heat and at white heat. The 

 first method of experimenting was as follows : bars of soft iron, 1 cm. 

 in diameter, were placed so as to form the armatures of the electro- 

 magnets ; a coil of fine wire, the induction coil, was slipped upon these 

 bars, forming the armatures, and the curve was drawn, whicli i^howed 

 the distribution of magnetism over the armature when the electro- 

 magnets were excited ; then the bar was heated, and the change in the 

 curve noted. The induction coil was so placed that its plane was at 

 right angles to that of the coils of the exciting electro-magnets. This 

 apparatus showed a sligiit increase of magnetic susceptibility in the 

 bars of soft iron as they were heated. The magnetic state increased 

 up to the point when the bar began to change slightly in color from 

 the effect of the heat ; it then remained constant. Owing to the difii- 



