on the T/iermo-electric Qualities of Metals. 323 



of thermo-electric currents in the same metal when one part of it is left 

 unstrained, and the other is 



(1) Permanently affected by application and removal of longitudinal 



stress ; 



(2) Permanently affected by application and removal of lateral 



pressure ; 



(3) Under a longitudinal stress (a) within its limits of elasticity, 



and (b) beyond its limits of elasticity ; 



(4) Hardened by twisting ; 



(5) Annealed. 



3. He showed that for iron and copper permanent longitudinal 

 extension gave the same effect as permanent lateral contraction ; and 

 that this effect for both was opposite to that experienced by them 

 when under a stress which caused a temporary strain. Thus for a 

 copper wire under a longitudinal stress the current was from the 

 strained copper to the free copper across the hot junction, and the 

 magnitude of the current increased with the increase of the longitudinal 

 stress. If the stress were removed and the wire left with a permanent 

 strain, the current was now from the free copper to the strained 

 copper through the hot junction. Similar results were got with iron, 

 only the direction of the current was in each case opposite to the 

 direction of the current in the corresponding case for copper. The 

 highest temperature used in these experiments was about 100 C. 



4. A summary of Lord Kelvin's results is given on pages 296 and 

 297 of vol. 2 of his ' Mathematical and Physical Papers.' His results 

 for copper are given, for example, in the following table : 



Condition of strained conductor. Direction of 

 Conductor. current through hot junction is from 1 to 2. 



f 1. Under longitudinal traction. 



1. Soft. 



2. Permanently elongated by longitudinal 



traction, and left free from stress. 



1. Soft. 



2. Hammered transversely. 



rl. Annealed after being made brittle by 

 Bound copper I * 



+ twisting. 



1 2. Made brittle by twisting. 



/ 1. Annealed. 

 " 1 2. 



Suddenly cooled. 



5. To determine the magnitude of the thermo-electric effects obtained 

 from any one metal, strained and unstrained, was the object I had in 

 VOL. LXIV. 2 c 



