on the Length and the Resistance of Iron Bars. 455 



Position of the spirals. 



Magnetism. 



Extension. 



46-62 



3-7 



44-09 



5-8 



43-91 



5-8 



42-30 



4-6 



40-63 



3-5 



36-81 



0-9 



30-81 



3-3 



4 in the middle 



2 in the middle, 2 at the free end ... 



4 at the free end , 



Distributed 



2 in the middle, 2 at the fixed end ' 



4 at the fixed end 



Two at each end 



These numbers show distinctly how considerable is the part 

 which the mechanical pull exerts in the extension of the bars. In 

 the position c the extension is much greater than in a, although 

 the momentum of the bar is less. In g the extension is almost 

 the same as in a, although the momentum is a third less. In/ 

 the extension is inconsiderable, although the momentum is 

 greater than in g. Buff has also called attention to the coope- 

 ration of this mechanical pull. 



If, then, it has not only been shown that magnetizing lengthens 

 the bars, but also that that lengthening more especially comes 

 into play which tends to remove the molecules apart, it seems as 

 if the increase in resistance observed by Thomson might be very 

 easily explained. But the experiments with stretched wires do 

 not agree with this. For these, the arrangement formerly used 

 was applied without any change. The wires were surrounded 

 by a tube through which a current of water passed. I obtained 

 the following changes in length with three different wires (the 

 plus sign indicating an elongation, the minus sign a contrac- 

 tion) : — 



No. 



Wire. 



L. • d. 



Tension. 



Change 

 in length. 



7 

 8 



Hard 



mm. 

 1250 



mm. 

 1-3 



0-28 

 0-24 



grms. 



1000 



1000 

 20000 



1000 



1000 

 300 

 500 



1000 

 100 

 300 



1000 

 100 

 300 

 500 



1000 



divisions. 



_ p 



-4-7 

 -0-3 

 -0-2 

 -0-9 

 + 10 





 -0-2 

 -10 

 -0-9 

 -0-8 

 -01 





 -01 

 -0-4 





Hard 



1250 







9 





1250 



Annealed 



Covering dissolved off in sulphuric acid. 



_ 



