206 Mr. C. A. Oarus- Wilson on the Behaviour 



In Experiment I. the range of stress referred to in (iii.) is 

 about 15 to 22 tons per square inch, the latter being the 

 yield-point ; and in Experiment III. it is about 10 to 15 tons 

 per square inch, the latter being the yield-point. In Experi- 

 ment II. the critical stress appears to coincide very nearly 

 with the stress at the yield-point. 



It is interesting to compare these results with those obtained 

 by Joule. In his ' Scientific Papers/ pp. 253-256, Joule 

 shows that an iron or steel bar elongates on being magnetized, 

 but that if the bar is strained, the effect is reduced until, at 

 a certain load, no effect is observed, and if the load be further 

 increased, magnetization produces a shortening of the bar. 

 These experiments were made with soft iron and steel wires. 



Joule's experiments show that magnetization produces a 

 minute elongation of a bar, those quoted above show that 

 mechanical stress increases the magnetization of a bar, i. e. 

 mechanical stress produces a change similar to that pro- 

 duced by magnetization. It would seem, then, that Joule's 

 critical point of no elongation (-f or — ) must be reached 

 when the molecular elongation produced by the mechanical 

 stress is the same as that which would be produced inde- 

 pendently by a certain given intensity of magnetization. For 

 at that point, the molecular elongation having already been 

 produced by mechanical stress, the effect of the given magne- 

 tization would be nil, i. e. there would be no further elonga- 

 tion (+ or — ). Now, taking an intensity of magnetization, 

 denoted by 5 in Joule's experiments, we see that this produces an 

 elongation in a 12 inch length of soft iron wire (0*25 in. diam.) 

 of 150^60 in.; assuming Young's modulus for the iron at 13 x 10 3 

 tons per square inch, the mechanical stress required to pro- 

 duce this elongation would be about 0*03 ton per square inch. 



According to Joule's experiments the critical stress is at 

 about 6 tons per square inch, i. e. in order to produce this 

 molecular elongation by mechanical means we require an 

 elongation of the bar 200 times as great as that which accom- 

 panies the same effect produced by magnetization. 



We are thus dealing with two distinct kinds of elongations — 

 firstly, that produced by a relative motion of the molecules ; 

 and secondly, that produced by a straining of the molecules 

 themselves. 



It is clear that if the atomic displacement should be perma- 

 nent, there would be a permanent change in the physical and 

 chemical properties of the iron ; and as there certainly is a 

 change produced by permanent set, it is highly probable that 

 we must look to such a straining of the molecule to account 

 for it. 



