Strain on the Thermo-electric Qualities of Metals. 169 



§ 7. The thermo-electric difference between glass hard tempered 

 steel, annealed steel, and unannealed steel, was found by similar ex- 

 periments to be : — 



1. Glass hard steel 1 , ^ 



2 Unannealed steel 1 ^'^ "^^^^^^^^^^^'^ P^^ ^^g^^^ 



1. Unannealed steel ~\ ^.-j ^ 



2. Annealed steel... J 



1. Glass hard steel 



2. Annealed steel 



' j7-67 



The direction of the current through the hot junction was in ever}^ 

 case from hard steel to soft steel. 



B. — Thermo-electric difference hehveen free wires and wires previously 

 permanently elongated by longitudinal stresses (§§ 8 — 10). 



§ 8. Attempts were now made to determine the thermo-electric 

 difference between free wires and wires previously permanently elon- 

 gated by a longitudinal stress. It was found difficult to elongate the 

 hard wires permanently to any appreciable extent before they broke. 

 Several methods for stretching the wires were tried, and the method 

 finally adopted, was to take two pieces of stout copper rod, bent into 

 the shapes shown at A and B in fig. 2, and to wind the wire to be 



Fia. 2. 









r — i 









stretched several times round A and B. The end A was clamped in a 

 fixed vice and the end B fixed in the clamp of a screw arrangement. 

 By turning the screw the wire was stretched tight. Two ink marks 

 were then put on the wire at G and D 60 cm. apart. The screw was very 

 slowly turned, and the distance between C and D measured until the 

 necessary elongation was produced or until the wire broke. The wire 

 generally broke where it lay tangentially to either rod A or B. 



§ 9. The greatest percentage permanent elongation that could by 

 this method be got in hard drawn copper, manganin, nickel, and 

 German silver, was 0-7, 0-5, 0-7, and 0*5 respectively. The thermo- 

 electric difference between the stretched and the unstretched wires was 

 then determined, as described in § 3, and the results are given in 

 Table IV. 



