ELECTRICAL CONVECTION OF HEAT. 



277 



The straight lines, which represent the thermo-electric powers 

 of the different metals, are unequally inclined to the axis of tem- 

 peratures; they cut this axis at the point corresponding to the 

 temperature of the neutral point with the metal compared, and their 

 inclination to the axis is the specific heat of electricity corresponding 

 to each of the metals. 



285. ELECTRICAL CONVECTION OF HEAT. It was important to 

 verify experimentally the hypothesis which serves as the basis for this 

 theory that is to say, the existence of changes of potential due to 

 changes of temperature. The method employed by Sir W. Thomson 

 consisted in establishing that, by the passage of a current, reversible 

 calorific effects are produced analogous to the Peltier effect. 



Let us consider a bar of metal (iron, for instance), the central 

 part of which AA' (Fig. 70), is kept at a constant temperature T, 

 while the ends B and B' are kept at o. 



Fig. 70. 



A' M' B' 



The distribution of temperatures is represented by the curves 

 BDD'B'; so long as the current does not pass, the distribution is 

 obviously symmetrical, and is represented by curves such as BPD 

 and D'P'B'. The passage of a current produces two effects at each 

 point ; firstly, a heating regulated by the law of Joule ; secondly, a 

 disengagement or an absorption of heat produced by the fixed fall of 

 potential corresponding to the difference of the temperature of two 

 adjacent points. 



If we only take into account Joule's law, the distribution of 

 temperatures is still symmetrical, and may be represented by the 

 dotted lines BQD, D'Q'B'. 



The second effect is reversible with the direction of the current. 

 If the current goes from left to right with the arrow, there will be a 

 fresh heating in the anterior part of the wire BA, where the tem- 

 peratures increase, and a cooling in the posterior part where they 

 decrease. The distribution of temperatures is then dissymmetrical, 

 and may be represented by the dotted curves BRD, and D'R'B'. 



