184 Mr Laws, Experiments on the Thomson Effect 



where r is the resistance per unit length, and both G 2 and r are 

 measured in C.G.S. units. 



Or H 2 = C<?r x 10 7 ergs 



when G and r are expressed in amperes and ohms respectively. 



Hence if S lt S 2 are the changes in the compensating currents 

 observed in the two cases these are proportional to the correspond- 

 ing changes of temperature, and 



H 2 S 2 /2 2S 2 ' 

 ™ du 

 1 dx „ 8j 



or 



10 



= H t = ~ . C, a r x 10 7 . 



^o 2 



That is, a 



G->: r Si ., ' ,on 



Y ergs per C.G.S. unit current per 1 U. 



10 8 



T ' C x * dd/dx 



The validity of the above assumption that the heat produced 

 is proportional to the change of temperature observed is evident 

 from the following data for the rise in temperature due to different 

 currents in the rods. 



It will be seen that within the errors of experiment the rise in 

 temperature is proportional to the square of the current or, by 

 Joule's law, to the heat developed. 



In practice it was found that the heat produced at a given 

 point by a current in the rod when this was at an uniform 

 temperature throughout was not altogether independent of the 

 direction of the current, or that something analogous to a Peltier 

 effect occurred throughout the length of the rod. 



Hence in these experiments the change in temperature pro- 

 duced by a reversal of the current is measured both when there is 

 a temperature gradient along the rods and when these are at the 

 same temperature throughout, and the latter applied as a correction 

 to the former. 



Another series of observations has then been made with the 

 rods reversed. 



To obtain the temperatures at various points along the rods a 

 number of thermo-couples of copper and German-silver were 



