438 BURGESS AND KELLBERGI CRITICAL RANGES OF IRON 



described in Reprint 213 (99.98 per cent iron). The design of the fur- 

 nace and heating circuits were such that the rate of heating could be 

 exactly controlled and the temperature of the iron was constant over 

 its length at any instant. The Wheatstore bridge with which the 

 best series was taken is one designed by E. F. Mueller of this Bureau, 

 it being a modification of the one described in B. S. Reprint 124, in which 

 are also described the methods of use of the resistance pyrometer. 

 The precision of the resistance measurements was better than 0.00001 

 ohm and of the time 0.1 sees., or equivalent to 0.005°C. in tempera- 

 ture differences and to 1 in 1,000,000 of the iron resistance at 800°C. 

 This is some 1000 times the precision of Honda and Ogura. 



Pure Xron 

 Pes/'stance us Temfierafure 

 o o o Heating 

 *■ ♦ + Coo/inq 



soA/ao 



"r Temp. ^rCent ^f 



• 750^ aoo' 



Fig. 2. Resistance vs. temperature curves of pure iron. 



In all, six iron thermometers were used from three samples 

 and all gave the same characteristics for pure iron. In figure 2 

 are shown the observations of the second heating and cooling 

 curves taken with thermometer F-6, which are typical of the 

 behavior of iron, and in figure 3 the temperature-coefficient of 

 electrical resistance of pure iron, or more exactly the ratio of 

 change of resistance of iron to that of platinum with temperature.^ 



. f Tables are given in the complete paper to be published by the Bureau of 

 Standards of the actual observations from which these curves are drawn. 



