PEIRCE. — RESISTIVITY OF HARDENED CAST IRON. 



197 



before was 831 and boiling reduced this to 740. The same magnetized 

 castings are tested year after year in the Jefferson Laboratory, and so 

 far as my experience goes, a properly hardened and aged magnet made 

 of cast iron is quite permanent if it is exposed to such fields as that 



W 



s 



M 



• N. 



Figure 4. 



of the earth, and mechanical shocks do not injure them in any way, 

 if the metal is not broken or abraded. 



Although a knowledge of the resistivity of a piece of cast iron tells 

 very little about its temper unless one knows also its resistivity in the 

 annealed state, yet the resistivity of different portions of the same 

 piece is a trustworthy measure of the uniformity of temper. Tried by 



