Trouton — On Thermo-electric Current in Single Conductors. 173 



We may picture what occurs when the high temperature spreads 

 out along the wire somewhat as follows : — When the temperature 

 of a portion is raised to a bright heat, let us suppose the structure 

 to be altered, and with it the electrical potential. Let mn represent 

 the wire, and the ordinates of the curve the temperature at each 

 point. Then, for simplicity, let us for the moment suppose that the 

 difference from the altered part to the unaltered is sudden in the 



\ 

 \ 



AB 



BA 



B'A' 



wire and not gradual. Say, at AB and again at BA, so that from 

 A to B there is a difference in the potential, and again of the same 

 amount, but in the opposite sense at AB. Now, if the temperature 

 spread out on one side, as represented by the dotted line, the junc- 

 tion BA will go out to B'A'. However, if we suppose the high 

 temperature to travel out faster than the junction, while the junc- 

 tion- is behind its final position, it is at a higher temperature than the 

 junction at AB; and there will be a current, flowing from m to n, 

 if the potential of the centre part was originally higher than the 

 rest of the wire. The reverse occurs when the high temperature 

 goes back. The junction follows slower and is at a lower tempe- 

 rature than AB until it arrives at the final position. The fall in 

 the potential is meanwhile less than at AB. So the current in this 

 case flows from n to m. Now, if we suppose a great number of these 

 junctions beginning with the unaltered wire, and ending with the 

 completely altered, each will have its own normal temperature and 

 can act just as described above in the case of one. By supposing a 

 sufficient number of these we have at length a continuous alteration 

 in the structure of the wire, which is what probably takes place. 



1 



In the case of the moving flame the junction in front is at a 

 higher, and the one behind at a lower temperature, than the tern- 



