CONTRIBUTIONS FROM THE JEFFERSON PHYSICAL LABORATORY 

 OF HARVARD UNIVERSITY. 



ON THE PROPERTIES OF MAGNETS MADE OF 

 HARDENED CAST IRON. 



By B. O. Peirce. 



Presented January 11, 1905. Received February 8, 1905. 



During the last six or seven years a large number of d'Arsonval gal- 

 vanometers, in which the permanent fields are due to hardened and 

 artificially seasoned cast-iron magnets, have been used in the Physical 

 Laboratory of Harvard University, in competition with similar instru- 

 ments furnished with hardened forged-steel magnets from the shops of 

 well-known makers. For nearly five years also magnets of the same 

 kind have been employed in standard mirror amperemeters and voltmeters 

 fixed in the laboratory, in cases where it was desirable that the indica- 

 tions of the instruments should be trustworthy within one part in a 

 thousand of their larger deflections, over a considerable range of room 

 temperatures. Besides the cast-iron magnets which we have made our- 

 selves, we have of late used a number of others in moving-coil galvanom- 

 eters purchased in the market — some of the best of them from the Leeds 

 and Northrup Company. 



It early appeared from tests made on these instruments, that whereas 

 good iron castings as they come from the foundry make most unsatis- 

 factory magnets, so far as permanence is concerned, magnets made of 

 castings properly hardened and aged after being machined — if ma- 

 chining is necessary for the purpose to which the magnets are to be put 

 — compare favorably in strength, in permanence, and in the relatively 

 small changes of their moments with room temperature, with tlie best of 

 tool steel magnets, even if in strength, though not in their other qualities, 

 they fall a little behind magnets made, in a forming press, of steel 

 specially prepared for the purpose. 



Although chilled cast-iron bar magnets have been used for a long 

 time in a few forms of telephones, it is usually best to make straight 

 magnets (which do not need to be hammered), of steel ; but the forging 



