of Cold Hard Steel. 



489 



tempered. So much of the excess of glass-hard temper 

 vanishes even at 100°, while the rod retains hardness and 



Curves 66°, 100°, 185°, 330°, 1000° show the reduction of the specific 

 resistance of steel due to continued tempering at these temperatures. 

 The abscissas are minutes of exposure. 



Curve "secular"' shows the corresponding secular softening of steel. The 

 abscissas are months. 



coercitive force, that we based our method of making per- 

 manent magnets*' on these observations. It is upon the 



* Temper the glass-hard steel rod to the limit for 100° ; then magnetize 

 to saturation ; finally temper the magnet to the magnetic limit for 100°. 



