436 BURGESS AND KELLBERG : CRITICAL RANGES OF IRON 



PHYSICAL CHEMISTRY — The electrical resistance and criti- 

 cal ranges of pure iron. G. K. Burgess and I. N. Kell- 

 BERG, Bureau of Standards. 



The exact location and description of the critical ranges A2 

 and A3 of pure iron, determined by heating and cooling curves, 

 has recently been published by the Bureau of Standards. ^ Dr. 

 Benedicks of Stockhohn has since carried out dilatation measure- 

 ments- which show that A 2 is accompanied by an expansion 

 change, hitherto undetected. Messrs. Honda and Ogura,^ 

 following a number of other experimenters,'' have plotted the 

 magnetic and resistance-temperature curves for pure iron over 

 the range 0° to 1000°C. Although their observations appear 

 to give the general trend of the resistance-temperature curve 

 of pure iron, they do not give an exact representation of the 

 Tesistance changes taking place at A^ and A?., mainly for lack 

 of sensitiveness. 



In view of the importance of the subject and as providing a 

 part of an adequate experimental basis for the elucidation of 

 the question of the allotropy of iron, it was thought worth while 

 to make as exact a determination of the resistance-temperature 

 relation of pure iron as the experimental means at our command 

 permitted, paying particular attention to the form of. the curve 

 over the A2 and A3 critical ranges. 



The experiments here described were begun in the suinmer 

 of 1912 and several preliminary methods of experimentation 

 were tried out before satisfactory sensitiveness, accuracy, speed 

 in manipulation, and closeness of observations to each other 

 were obtained. In some of the earlier work the method was 

 tried of bringing the heating bath or furnace to a definite tempera- 



^ G. K. Burgess and J. J. Crowe: Critical ranges A^ and A3 of pure iron, 

 Bureau of Standards Scientific Paper No. 213. 1914. Also Bull. Am. Inst. Mining 

 Tj]ngineers. October and December. 1913. 



2 Carl Benedicks: Experiments on allotropy of iron: behavior of ferro-mag- 

 sietic mixtures; dilatation of pure iron. Jl. Iron and Steel Institute. May, 1914. 



' K. Honda and Y. Ogura: tJber die Beziehung zwischen den Anderungen der 

 Magnetisierung und des elektrischen Widerstandes im Eisen, Stahl und Nickel 

 fcei hohen Temperaturen. Science Reports, University Sendai, 3: 113. 1914. 



* See Bureau of Standards Reprint No. 213, above cited. 



