[ 508 ] 



LV. On the Laws regarding the Direction of Thermo-electric- 

 Currents enunciated by M. Thomas. By Chakles H. Lees, 

 D.Sc, F.R.S., Professor of Physics in the East London 

 College, University of L^ondon * . 



IN number 8 o£ the Bidletin de la Classe des Sciences of the 

 Academie Ko.yale de .Belgique for 1909, M. Bruno- 

 Joseph Thomas enunciates the following laws as the result of 

 his examination of the direction of the electric current pro- 

 duced in more than 100 thermo-couples : — 



u 1. When a difference of temperature is established be- 

 tween the two junctions of a thermo-electric couple, the 

 current produced flows from the hotter to the colder junction 

 through the metal which is electrically the better conductor, 

 if the product of the coefficient of electrical conductivity of 

 that metal by the coefficient of thermal conductivity of the 

 other metal exceeds the product of the coefficient of thermal 

 conductivity of the first metal by the coefficient of electrical 

 conductivity of the second. 



" 2. If the former product is less than the second, the 

 current flows from the hotter to the colder junction through 

 the metal electrically the worse conductor. 



" 3. If the two products are equal no current is produced." f 



The author states further that at different temperatures 

 " for a given couple the strength of the current produced 

 increases with the ratio (" rapport ") of these two products, 

 and other things remaining equal, the strength will be pro- 

 portional to that ratio " {. 



M. Thomas supports his theory by 14 pages of tables, 

 in which the values of the products for each of the 105 

 couples which can be obtained by combining together in pairs 

 the 15 metals of Becquerel's thermo-electric series, are com- 

 pared. Not a single exception to the theory is found, and at 

 first sight it appears as if M. Thomas had succeeded in 

 establishing a most important and hitherto unsuspected law 

 of connexion between the thermo-electric properties of a 

 metal and its conductivity for heat and for electricity. On 

 more careful examination, however, it is seen that M. Thomas 

 in his tabular proof has based his argument entirely on the 

 order of the metals in BecquereFs thermo-electric series, on 

 the heat conductivities of 7 of the metals according to 



* Communicated by the Physical Society : read February 25, 1910. 

 t Bulletin, Classe des Sciences, Bog. Acad. Belg. 1909, p. 903. 

 \ Ibid. p. 925. 





