340 Von Tunzelmann on the Production of Thermoelectric 



In the arrangement finally adopted, there was attached to 

 the longer arm of the lever a tin can open at the top, and 

 having at the bottom a neck fitted with an india-rubber tube, 

 which could be closed by merely bending it up and hitching 

 it in a hook attached to the can for that purpose. The strain 

 on the wire was then produced as gradually as was desired, by 

 pouring in measured quantities of shot ; and it could be removed 

 as gradually by letting the shot run out by the india-rubber 

 tube. 



The two cans through which the wire passed were filled with 

 water, the water in the upper can being kept at the temperature 

 of 100° C. by means of a gas-burner, while that in the lower 

 can could be kept for a considerable time at a uniform tempe- 

 rature by allowing a current of water, of the same temperature 

 as the place of experiment, to circulate through it. 



The extremities of the experimental wire were bent round 

 in a large curve and brought close together; they were then 

 tied to the extremities of two covered copper wires connected, 

 through a four- way key, with a Thomson's galvanometer 

 having a resistance of between one and two ohms. The junc- 

 tions were then placed side by side separated by thin paper, 

 and wrapped up in cotton-wool, as was done by Thomson in 

 his experiments, to prevent the production of currents by the 

 unequal heating of the two junctions. 



Before being used the wires were annealed : — the iron and 

 steel wires by being heated to redness in an iron tube, through 

 which a current of coal-gas was passing to prevent oxidation ; 

 the copper wire by being slowly passed through a Bunsen 

 flame, as it was found that the exposure of the copper at a red 

 heat to the current of coal-gas produced an effect similar to 

 that known as over-poling in the process of refining copper, 

 the wire being rendered so brittle as to break with the least 

 strain. 



Thomson found in his experiments that when a weight was 

 applied so as to produce a state of strain in a portion of the 

 wire, and the two junctions of strained and unstrained por- 

 tions were kept at different temperatures, in iron and steel 

 wire a current was produced the direction of which was from 

 the unstrained to the strained portion across the hot junction, 

 while in copper wire the current was in the opposite direction. 

 When the weight was removed the result was in either case 

 a weaker current in the reverse direction. 



Some experiments of the same nature have also been made 

 by M. le Roux, and described in the Annates de Chimie et de 

 Physique, 4th series, vol. x. p. 201 (1867). He obtained 

 results of the same nature as Thomson — with the notable 



