ON THE TEMPERATURE COEFFICIENTS OF MAGNETS 

 MADE OF CHILLED CAST IRON. 



By B. O. Peirce. 



Presented January 14, 1903. Received January 14, 1903. 



Besides a number of d'Arsonval galvanometers, furnished with 

 hardened forged steel magnets, from the shops of well known makers 

 in America and in Europe, there are in the Physical Laboratories of 

 Harvard University about thirty similar instruments in which the per- 

 manent fields are due to chilled and seasoned cast iron magnets. These 

 latter have proved very satisfactory, and, after a trial of three years, we 

 are about to add to their number. 



Although chilled cast iron magnets were used years ago in some forms 

 of telephones, straight magnets are most conveniently made of steel ; 

 indeed if they are to be employed in measuring the intensity of the 

 earth's field, the best tool steel, ground slowly into shape under water 

 after the hardening, is not too homogeneous for the purpose. Steel for 

 permanent magnets, however, needs special skill in the handling, if the 

 results are to be satisfactory, and not every successful tool maker knows 

 how to forge and to harden, well and quickly, even a horseshoe magnet, 

 unless it be of very simple form. Two straight, hollow bar magnets 

 were made and ground most carefully for use in the Jefferson Physical 

 Laboratory, by a firm which manufactures machine tools of the highest 

 grade. These were supposed to be as nearly alike as possible, but they 

 proved to be magnetically very different, for the permanent moment of 

 one was twice that of the other. "• 



Of late I have been using magnets made of soft iron castings, subse- 

 quently chilled, to furnish the artificial field in an oil damped ampere- 

 meter, and in a similar voltmeter firmly set up in the laboratory, and, since 

 it was desirable that the indications of these instruments should be 

 trustworthy within one part in a thousand of their larger deflections, 

 it became necessary to test the permanency of the magnets, and to 



