44 MEASUREMENT OF ELECTROMOTIVE FORCES. 



An analogous calculation would be made for zinc sulphate, 

 starting from the numbers 53,500 for the heat of formation of zinc 

 sulphate ; the electromotive force of contact is then 



H' = 2 -309 -0-241 = 2 -068 volt. 



1034. MEASUREMENT OF THE THOMSON EFFECT. The Thomson 

 effect is analogous to the Peltier effect, and, like it, is reversible with 

 the direction of the current ; it only differs in the fact that the fall of 

 potential, instead of being localised at the surface of contact of two 

 different substances at the same temperature, is produced between 

 two portions of the same substance at different temperatures. We 

 have explained the manner (285) in which Sir W. Thomson has 

 verified the electric transport of heat, which follows as a necessary 

 consequence. 



Le Roux has measured this effect by an ingenious arrangement. 

 Two bars of the same metal, AB, A'B' (Fig. 212), are arranged 



parallel to each other. The ends A and A' are kept in a bath at 

 zero, and the ends B and B', joined by a plate of copper, in a bath 

 at 100. When the ends A and A' are connected with a strong 

 battery, the temperature of the intermediate points is modified in 

 accordance with Joule's law, and with the electric transport of heat. 

 The difference in temperature of two similarly situated points 

 C and C', which would correspond to twice the Thomson effect, 

 is measured by a thermoelectric pile between the two bars. If the 

 symmetry of the apparatus were complete, the galvanometer G of 

 the thermoelectric circuit should be at rest if the current in the rods 

 is suppressed; but this condition is not necessary, for, if the direction 

 of the current is reversed, the difference of the deflections observed 

 in the galvanometer corresponds in all cases to four times the 

 Thomson effect. The experiment may be recommenced by re- 

 placing one by the other bath at constant temperature, and the mean 

 of the results eliminates all defects of symmetry. 



