PEIRCE. — MAGNETIC BEHAVIOR OF HARDENED CAST IRON. 359 



it is not easy to harden a long slender rod of cast iron without its be- 

 coming slightly crooked in the process. An isthmus piece of cast iron 

 . has, therefore, to be ground into shape at much labor, from a glass hard 



TABLE V. 

 Cylinder op Soft Crescent Drill Rod. 



cylinder. The hardened steel isthmus pieces, on the contrary, were 

 shaped while soft, and were then chilled inside a supporting tube after 

 they had been heated in a gas furnace. 



TABLE VI. 

 Isthmus of Soft Crescent Drill Rod. 



The " Isthmus Method " for determining the permeability of a small 

 piece of magnetic metal at a high excitation rests, of course, upon the 

 assumption that the value of H just without the test piece is equal to 



