CONTRIBUTIONS FROM THE JEFFERSON PHYSICAL 

 LABORATORY, HARVARD UNIVERSITY. 



THE RESISTIVITY OF HARDENED CAST IRON AS A 



MEASURE OF ITS TEMPER AND OF ITS FITNESS 



FOR USE IN PERMANENT MAGNETS. 



By B. Osgood Peirce. 



Presented May 11, 1910. Received August 26, 1910. 



It has long been known that the specific electrical resistance of a 

 piece of soft tool steel is materially less than that of the same piece 

 after it has been hardened, and that the relaxing of the temper of any 

 piece of steel or iron makes the specific resistance less ; but the first 

 systematic study of this phenomenon was made by Messrs. Barus and 

 Strouhal whose work is summarized in Bulletin 14 of the U. S. Geolog- 

 ical Survey.^ 



In one experiment which these gentlemen made upon rods of " Eng- 

 lish Silver" steel, 0.15 cm. in diameter and all originally glass-hard, 

 different pieces were tempered by heating them to different fairly high 

 temperatures, as indicated by the oxide tints on their surfaces, and were 

 then cooled. When the specimens thus treated were tested it appeared 

 that the harder the temper, the higher was the specific resistance (s) 

 referred to the centimeter cube, and the lower the temperature coeffi- 

 cient (a) of the specific resistance. In the case of a certain glass-hard 

 rod, s, in microhms, was 45 and a was 0.0016 ; while in a thoroughly 

 annealed rod of the same lot, s was 16 and a about 0.0040. From these 



^ Wiedemann, Lehre von der Elektricitat, Vol. I, p. 502 ; Benoit, Comptes 

 Rendus. 76, 342 (1873) ; Matthiessen and Vogt, Pogg. Ann., 122, 10 (1864) ; 

 Auerbach, Wied. Ann., 5(1878); Wied. Ann., 8,479 (1879); Callendar, Phil. 

 Tran.s. 18S7; Strouhal and Barus, Wied. Ann., 11, 976 (1880) ; Barus, Phil. Mag., 

 8, 341 (1879) ; Chemoff, Vortrag gehalten in der Russischen Technischen Gesell- 

 schaft, 1868; Jarolimek, Dingler's Journal, 221, 436 (1876); Jarolimek and 

 Ackermann, Zeitschrift fiir das Chemische Grossgewerbe, 1880; Percy-Wed- 

 ding, Ei.senhiittenkunde, II, p. 130, 1864; Karsten, Karsten und von Dechen's 

 Archiv, 25, 223 (1853); Barus and Strouhal, Bulletin of the United States 

 Geological Survey, No. 14; Caron, Comptes Rendus, 56, 43 (1863); Barus, 

 Physical Review, 30, 348 (1910). 



