1884.] 



Explanation of HaWs Phenomenon. 



345 



therefore, a current is passed from a stretched portion of a wire to a 

 compressed portion, heat will (according to the laws of the Peltier 

 e ffect) be absorbed at the junction if the metal is copper, and will be 

 developed at the junction if the metal is iron. In passing from com- 

 pressed to stretched regions the converse effects will occnr. 



Let us now imagine the metal plate to be divided into four equal 

 regions, A, B, C, D as shown in fig. 4. Let a current pass through 

 the plate from E to F and let a force (produced electromag- 

 netically or otherwise) be applied in the direction HG. First, 

 suppose that the plate is of copper. Then the current travelling 

 from E to the line OG passes from a compressed to a stretched 

 portion of the metal ; heat will, therefore, be developed in the 

 region A. Between the line OGr and the point F, the current 

 passes from a stretched to a compressed portion ; heat will, therefore, 

 be absorbed in the region B. For similar reasons heat will be 

 absorbed in C and developed in D. The temperature of the copper 

 plate will, therefore, not be uniform ; the portions A and D being on 

 the whole hotter than the portions B and C. But the resistance of a 

 metal increases with its temperature. The resistance of A and D 

 will, therefore, be greater, and the resistance of B and C smaller than 

 before the plate was subjected to the strain. If, therefore, GH were 

 originally an equipotential line, it is clear that it will be so no longer. 

 An equipotential line through the point will now be inclined to 

 GH in the direction KL as shown in the figure.* 



Fig. 4. 



Compressed portions are shaded. Stretched portions are black. 



Supposing the plate to be of iron instead of copper, the thermo- 

 electric effects will be reversed, and the regions which in the former 

 case were hot, will now be cold, and vice versa. The distribution of 



* The displaced equipotential lines KL, MN, will in point of fact probably be 

 curved, and not straight as drawn. 



