124 Changes in Length in Iron Wires hy Magnetization. 



for weak fields and slight tension, the wire shortens when the 

 field is removed. But in these cases the residual induction 

 n the wire is always much less than the induction with the 

 field on. And finally the elongation with the field off 

 becomes as great or even greater than the maximum elonga- 

 tion with the field on. After this the length tends to remain 

 constant until the end of the experiment, although there is 

 a very slight shortening- for the strongest fields. This per- 



dl 

 inanent lengthening for the soft iron gave a value of -y as 



great as 33 X 10~~ 7 for the least tensions used. The straight 

 portion of the curves with field on shows a contraction that 

 is much more rapid than for annealed piano-wire, and its 

 slope is more than twice as great as that for the natural 

 piano -wire. 



Increased tension showed exactly the same effects as upon 

 the annealed piano-wire, though the amount of the changes 

 due to tension was much greater. 



The complete series of experiments would tend to confirm 

 Joule's statement that the changes in length caused by mag- 

 netization are greater in proportion to the softness of the 

 iron. This is more marked in the initial elongations than in 

 the final contractions. 



The elongation curves, both when plotted to magnetizing 

 force and when plotted to induction in the iron, seem to 

 show an elongation due to the magnetic induction in the wire 

 and a contraction caused by the magnetizing field, directly 

 proportional to the field in value. In all cases for a definite 

 induction the wire was longer with the field off than with 

 the field on. 



More than one application of the same magnetizing field 

 gave slightly cumulative effects. Repeating any reading, 

 thus, made the change a very little greater in the direction 

 of the original change, but the increase was not significant. 



0/ O 



The changes in length caused by adding and removing a 

 definite weight both before and during magnetization gave 

 no evidence of a change in Young's modulus, due to mag- 

 netization. But an examination of all the data of the complete 

 series of experiments suggests an increase in Young's 

 modulus with the intensity of magnetization, the maximum 

 change not exceeding one-half per cent., and nearly all of it 

 having occurred before the magnetization curve turns towards 

 the horizontal. This view fully explains the reduced elong- 

 ation and greatest final contraction with the larger tensions. 



