510 Prof. C. H. Lees on the Laws regarding the 



thermocurrent flows from hot to cold in the \ De " er \ 



(worse j 



electrical conductor. That is, the current flows from hot to 



, , . ,, , , p i . i electrical conductivity 



cold in the conductor tor which — n — = — —— — J - 



is the greater. heat conductivity 



In the original form the law was tested by a comparison 

 of the 210 products of the electrical and heat conductivities 

 of pairs of the 15 metals ; in its present form it may be 

 tested by a comparison of the 15 quotients of the electrical 

 by the heat conductivities of the 15 metals. 



It may be noted that the electronic theories of conduction 

 of heat and electricity in metals make this quotient the same 

 for all metals, and that these theories and that of M. Thomas 

 are therefore incompatible. 



In selecting the experimental material with which to test 

 M. Thomas's theory it is important to remember that both 

 the electrical and the thermal conductivities and the thermo- 

 electric properties of metals are influenced by the presence 

 of impurities, and that it is advisable to use only results 

 obtained with pure materials. If, however, results for abso- 

 lutely pure materials are not available, those for approximately 

 pure materials may be used, if in the case of any particular 

 metal the impurities in the samples used in the different sets 

 of observations were the same in amount and in material, or 

 better if the whole of the measurements have been made 

 on the same material. Fortunately in the present case we 

 have the experiments of Jager and Diesselborst *, who 

 determined the quotient of the thermal by the electrical 

 conductivity and the thermo-electric power of each of a 

 number of rods of very nearly pure metals. Their results 

 for the quotients of the two conductivities have been con- 

 firmed by more recent observations made by other methods f, 

 while their results for the thermo-electric powers of the 

 metals agree with those obtained previously for approximately 

 pure metals J. 



They may be used therefore without hesitation as the best 

 results available at the present time for the comparison of 

 M. Thomas's theory with experiment. 



* W. Jager and H. Diesselhorst, Wissenschaftl. Abhandl. der Phys.- 

 Techn-Reichsanstalt, iii. p. 270 (1900). 



t C. H. Lees, Phil. Trans. Roy. Soc. Lond. A. 208, p. 381 (1908). 



X K. Noll, Ann, der Physik, liii. p. 874 (1894) ; W. H. Steele, Phil. 

 Mag. xxxvii. p. 218 (1894). 



