428 



Dr. S. Bidwell. On the Changes of 



[Apr. 11, 



changes of length undergone by an iron wire when loaded with 

 weights of 351 and 1600 kilogrammes per sq. cm., the curve for the 

 first-named load being practically the same as for an unloaded wire. 

 Curves (F) and (G) indicate the changes of thermoelectric force with 

 field for an iron wire, first when loaded with 1620 kilogrammes per 

 sq. cm., and afterwards without load, the points of observation being 

 marked by dots. The crosses on (G) and (F) indicate the course of 

 the curves (e) and (/) after correction for mechanical stress; the 

 correspondence is so close that it was impracticable to draw the 

 corrected curves separately. The vertical scale of the corrected curves 

 is not the same in the two cases ; that for the upper one is given in 

 the right-hand margin, and that for the lower in the middle of the 

 diagram. Data for the construction of these curves were obtained in 

 the same manner as before, and are given in the annexed table : — 



Table III. 



P = tractive force in grammes per sq. cm. E = elongation. 



H. 



P. 



P/200. 



E, 



E, 



p/200 + rc, 



P/200 + E, 



Carve (e). 



Curve (/). 



Curve (e). 



Curve (/). 



40 



6.550 



330 



26 



- 3 



59 -0 



30 -0 



50 



7,000 



35 -0 



28 



- 4 



63-0 



31 -0 



70 



7,700 



38 -5 



27 



- 6 



65 - 5 



32-5 



100 



8,500 



42 -5 



23 



-11 



65-5 



31 '5 



120 



9,000 



45-0 



20 



-14 



65 -0 



31 -0 



150 



9.500 



47 '5 



16 



-18 



63 -5 



29 -5 



200 



10,250 



51 -0 



10 



-25 



61 -0 



26 -0 



300 



11,100 



55 "5 



- 2 



-40 



53 -0 



15-5 



375 



11,500 



57 -5 



-10 



-52 



47-5 



5-5 



It is to be noted that the three sets of experiments — for change of 

 thermoelectric force, change of length, and lifting power — were made 

 with three different samples of iron wire. 



The effect of annealing upon the changes of length and of thermo- 

 electric force is illustrated in fig. 6. Curve (g) shows the elongation of 

 a wire in the state in which it was bought,* while (h) indicates its 

 behaviour after it had been carefully annealed. This operation was 

 performed by enclosing the wire in an iron tube which was placed in a 

 hot fire and allowed to cool gradually as the fire died out. Curve (H) 

 shows the changes of thermoelectric force in a piece of the same kind 

 of wire when in its original state, and (K) the modification which 

 resulted from heating the wire to redness in a Bunsen flame and 

 cooling it in air. In both cases the effect of annealing is to depress 

 the curve. 



* 'Roy. Soc. Proc.,' vol. 55, p. 230, fig. 1, 1891. 



