PETRCE. 



RESISTIVITY OF HARDENED CAST IRON. 



195 



measurements recorded in this table is very striking. The differences 

 are not greater than one might expect to find in a number of rods of 

 fine polished drill rod from the same lot. For the present discussion 

 it is of interest to notice that the permeabilities of the hard rods seem 

 not to be connected in any obvious way with the resistivities. For any 

 single specimen of cast iron, however, it is well known that hardening 

 usually decreases the permeability especially at comparatively low 

 excitations, and Figure 1 shows a rough kind of hysteresis diagram 

 which I obtained some years ago for a cast-iron frame of several 

 kilograms weight. Curve A corresponds to the soft state and B to the 



TABLE VIII. 



hardened state of the same piece of iron. At high excitations the 

 difference is not so striking but is very real. 



Table VIII gives approximately the results of some measurements 

 made two or three years ago upon cylinders and isthmuses of a cer- 

 tain kind of cast iron from the Broadway Iron Foundry. It must 

 be clearly understood, however, that this applies only to iron which 

 has once been through the annealing and subsequent hardening. 

 A repetition of the process makes the hardened iron mechanically 

 softer. As we have seen, a piece of cast iron properly hardened for 

 the first time makes as strong a permanent magnet as a piece of Stubs 

 Drill Rod does, but if the cast iron be several times hardened it be- 



