HALL. CONDUCTIVITY OF MILD STEEL. 271 



lNVBSTiaATI0N8 ON LiGHT AND HeaT, MADE AND POBLISHED WHOLLY Oa IN PART WITH 

 APPBOPaiATiON FROM THE ROMFOUD i'VVD. 



XV. 



ON THE THERMAL CONDUCTIVITY OF MILD STEEL. 



By Edwin H. Hall. 



Presented January 8, 1896. 



Some years ago,* the author gave an account of certain experi- 

 ments which he had made, by the method of Forbes, upon the thermal 

 conductivity of cast iron and cast nickel, and stated that he lioped 

 " soon to make trial of a method for estimating thermal conductivities 

 somewhat different from any heretofore used." This method has 

 now been tested, and it appears to be a good one, although certain 

 changes in the apparatus used, and presently to be described, might 

 be made with advantage. 



It is well known that early experiments upon thermal conductivity 

 were made with thin plates or sheets of metal. The results obtained 

 were absurdly erroneous, owing mainly to the difficulty of determin- 

 ing accurately the difference in temperature of the two faces of the 

 plates. Later the method was by various devices made to give better 

 results, but none satisfactory, and for a long time it has been discarded, 

 replaced generally by the method of Forbes, steady flow of heat in 

 a long bar exposed to the air, or that of Angstrom, waves of heat 

 sent along the bar at regular intervals. None of the methods have 

 been very simple, or easy of application, and the results obtained in 

 the case of the metal most studied, iron, have been as various as the 

 difficulty of the investigation would lead one to expect. 



Ev^ery one who has engaged in the experimental study of heat con- 

 duction, or has examined the curiously indirect methods employed in 

 this study, must have regretted the apparent impossibility of using 

 successfully the old method of thin plates. About four years ago, it 

 occurred to the author that one might determine the difference in 

 temperature of the two faces of a thin plate by making these faces 



* These Proceedings, Vol. XXVII. p. 262, 1892. 



