on the Length and the Resistance of Iron Bars. 457 



No. 



Wire. 



L. 



d. 



Tension. 



Alteration 

 in length. 







mm. 



mm. 



grms. 



divisions. 



10 



Annealed . . . 



2035 



024 



200 

 300 

 400 

 600 

 800 

 1000 



-0-2 

 -0-4 

 -1-8 

 -2-8 

 -3-3 

 -3-4 



11 



Covered 



2035 



0-36 



100 

 200 

 400 

 600 

 1000 



-01 

 -01 

 -0-2 

 -0-3 

 -10 





Annealed ... 







100 

 200 



800 



1000 



-20 

 -1-8 

 -1-5 

 -1-4 



The annealed wire 11 had already exceeded the maximum of 

 contraction at a tension of 100 grms. It is possible that it still 

 contained small bendings, which could only be removed by greater 

 tensions ; yet these bendings must always have occurred again 

 with smaller tensions, for the order of succession of the experi- 

 ments had no influence on the results. On the other hand, in 

 the case of wire No. 10 it is very distinctly seen how the mag- 

 netization always effects greater contractions with increasing 

 tension, as Joule found to be the case. 



The experiments with stretched wires lead definitely to the 

 conclusion that in magnetization there is an approximation of 

 the molecules ; and the supposition is obvious that there must be 

 a decrease of resistance during this process. To examine this 

 point, both the quadrangular pieces of brass through which the 

 wire was stretched were provided with binding-screws, from which 

 wires passed to a Siemens's resistance bridge. The bridge, as in 

 former experiments*, was connected with a small rheochord, by 

 which, when the measuring-wires had the ratio 10 : 1, a resist- 

 ance of 0*0005 could be estimated. Immediately after the alte- 

 ration in the length of the wire had been measured, its resistance 

 was determined ; the magnetizing current was then closed, and 

 the alteration of resistance observed. Such an alteration always 

 occurred, and it was found that the resistance of the iron was 

 always increased by magnetization. In each wire the increase 

 of resistance remained the same within the limits of observa- 

 tion, whether the tension was small or large, which could be 

 recognized from the fact that the scale was always displaced 

 the same number of divisions. There were no influences which 



* Poggendorflf's Annalen, vol, cxvii. p. 5. 



