Prof. Thomson on the Electro-dynamic Properties of Metals. 395 



It was also found that Aluminium must be neutral to either Pg, or 

 Brass, or Pg, at some temperature between —14° C. and 38° C. ; 

 that Brass becomes neutral to Copper at some high temperature, pro- 

 bably between 800° and 1400° ; Copper to Silver, a little below the 

 melting-point of silver ; Nickel to Palladium, at some high tempera- 

 ture, perhaps about a low red heat ; and Pg to impure mercury (that 

 had been used for amalgamating zinc plates), at a temperature be- 

 tween — 10° and 0°. Probably Pg becomes neutral to pure mercury 

 at some temperature below — 10° C. 



3. In the third part, eifects of mechanical strain, and of magnet- 

 ization on the thermo-electric qualities of metals, are investigated. 

 The author had previously communicated to the Royal Society* 

 results he had obtained regarding the thermo-electric qualities of 

 copper and of iron wires under longitudinal stress, namely, that the 

 former exhibits a deviation towards bismuth, and the latter towards 

 antimony, from the same metal in an unstrained state. 



The only kind of stress applicable to a solid which has no direc- 

 tional attributes, is uniform pressure or traction in all directions. 

 Hence it appeared probable to the author that a simple longitudinal 

 stress would induce different thermo-electric qualities in different 

 directions, in any homogeneous non-crystalline metal subjected to it. 

 But he had found (see Phil. Mag. July, 1854) that the thermo- 

 electric effect of longitudinal traction on a wire, either of iron or of 

 copper, is sensible to tests he could readily command, and more so 

 in the case of the former than in that of the latter. He therefore 

 made experiments to test the difference of thermo-electric quality 

 in different directions in a mass of iron under stress, and fully 

 established the conclusion that the thermo-electric quality across 

 lines of traction differs from the thermo-electric quality along 

 lines of traction, as bars of bismuth differ from bars of antimony. 

 The experiments he has already made nearly establish the conclusion 

 that unstrained iron has intermediate thermo-electric quality between 

 those of the two critical directions in iron under distorting stress. 



The experiments of Magnus show that wires hardened by wire- 

 drawing have different thermo-electric qualities lengthwise from wires 

 of the same substance softened by annealing. The author has veri- 

 fied, that in copper, iron, and tin, simple traction, leaving permanent 

 elongation, leaves also a thermo-electric effect, the same as Magnus 

 had found by wire- drawing, which is a composite application of lon- 

 gitudinal traction and lateral compression ; and that in a variety of 

 metals, namely, iron, copper, brass, tin, platinum, permanent lateral 

 compression (by hammering) leaves still the same thermo-electric 

 effect, as Magnus had found by wire-drawing. In cadmium, not 

 examined by Magnus, and lead, which had not a given result, the 

 experiments now adduced show a thermo-electric effect of hammering, 

 the same as in all the other metals except iron. Zinc wire was also 

 tested, and found to exhibit the same effect as copper, though 

 Magnus had found a reverse quality as due to wire-drawing. The 

 discrepance in this case is probably due to the peculiar effect of 

 * April 1854. See PhU. Mag. July, 1854. 



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