±66 Serviss — Internal Temperature Gradient of Metals. 



junctions is (except for the possible case of opposite gradients 

 in the insulation, which are probably very small compared with 

 that between the junctions) equal to that in the adjacent metal. 

 This method measures the gradient in the metal itself, while 

 Dr. Thwing's does not, although he states his results in that 

 way. But it is to be noted that any method working on gra- 

 dients in the metal is insensitive, because the large discontinuity 

 at the surface makes the gradient in the metal itself small. 



Suppose, on the other hand, that the outer junction be 

 placed in the air bath, as in Dr. Thwing's arrangement. It 

 would seem, at first sight, that this would be a much more 

 sensitive method for detecting internal generation of heat, 

 because while the inner junction is at the temperature of the 

 adjacent metal as before, the outer junction is supposedly at 

 the temperature of the air bath. This reasoning I believe is 

 dubious. Heat is still flowing down the wire, and must 

 now be radiated into the air from the junction itself through 

 its coating of asphaltum, and while we may well believe that 

 the inner junction is at, or very near, the temperature of the 

 metal at that point, and that the temperature of the wire as it 

 passes along the radius of the cylinder is not far from that of 

 the adjacent metal ; still the outer junction, cooling as it does, 

 is above the temperature of the air bath, due to the greater 

 discontinuity when cooling takes place with the poorer surface 

 conductivity through the insulation. This well known fact 

 must be borne in mind when, as here, a thermojunction is 

 exposed in air, and proper precautions must be taken to insure 

 that the true temperature of the surroundings is attained. 



In conclusion, I desire to express my sincere thanks to Pro- 

 fessor. E. H. Hall, who first suggested the work to me and who 

 made valuable suggestions at the beginning, and to Professor 

 B. O. Peirce for his kind interest and encouragement and the 

 generous loan of apparatus. 



Jefferson Physical Laboratory, 



Harvard University, 

 Nov. 11, 1907. 



