PEIRCE. — RESISTIVITY OF HARDENED CAST IRON. 



189 



Two test pieces from the Broadway Iron Works, Cambridgeport, 

 were annealed as they came from the foundry and then had resistivi- 

 ties 102.5 and 102.7 and a diameter of 0.574 cm. After both had been 

 hardened, the resistivities at about 20° C. were 122.5 and 122.0, and 

 after they had been again through the annealing furnace their resistivi- 

 ties were 108.7 and 107.1. The fourth time they were relaxed the 

 specific resistances were 112.6 and 112.6, and their average diameters 

 about 0.578 and 0.576. When they were finally hardened again, the 

 resistivities were 136.7 and 137.8 and both diameters were 0.581. It 

 did not seem worth while to carry the process farther. 



TABLE II. 



Cast Iron Rod four Times hardened and annealed. 



Another rod, presumably of a very different kind of iron, began with 

 a diameter of 0.574 and after four annealings had a mean diameter of 

 0.578. Its resistivity in the relaxed state rose in four steps from 93.9 

 to 102.5 5 the first time it was hardened its resistivity was 112.0, the 

 last time 116.5. 



In the three cases here mentioned the specimens would cut common 

 window glass easily the first time they were hardened ; they were 

 mechanically too soft to scratch the same glass when, having been 

 repeatedly hardened and relaxed, they were finally hardened so that 

 they had a higher resistivity than at first. 



Another rod from the same foundry had a resistivity of 102.0 when 

 it was first annealed, and a resistivity of 119.8 when it was hardened 

 for the first time. After an hour in steam at 100° this fell to 118.0, 

 and after five hours farther steaming to 116.6. The second time it was 

 annealed the rod had a resistivity of 106.5, and the third time of 

 107.2. 



