PEIRCE. — MAGNETIC BEHAVIOR OF HARDENED CAST IRON. 357 



many years. This spool kept its diameter practically unchanged during 

 the measurements here recorded, though it shrank very slightly soon 



TABLE I. 



Cylinder of Soft Cast Iron. 



after it was first made. The diameter of the wood was about 1.9135 

 centimeters, and that of the outside of the wire of the coil about 1.9591 

 centimeters, the last figure in each case being, of course, doubtful. 



TABLE II. 



Isthmus of Soft Cast Iron. 



Hard rubber is so susceptible in a magnetic field as to make it impos- 

 sible to use a spool of this material to support a testing coil. When 

 the specimen was in place between the jaws of the yoke, it was covered 

 by the shorter spools of the yoke. 



The value of H in the air just outside the metal was obtained by re- 



