30 DISPLACEMENT INTERFEROMETRY BY 



even the vibration telescope is in this respect unavailing. One would not be 

 justified in asserting that the observations fall very far off of the mean rate 

 indicated in figure 32, disregarding altogether the possibility of hysteresis. 

 The interesting feature of the diagram is thus the continuance of the contrac- 

 tions even in very strong fields. 



17. Theoretical observations. To account for such a graph as figure 32, 

 as a whole, one may argue that the initial elongations are to be referred to 

 the rotations of the molecular magnets. For these elongations are coextensive 

 in field variations with the marked increase in magnetization. It would then 

 seem further plausible that thereafter the attractions between the oriented 

 molecular magnets may be instanced to account for the persistent contractions 

 in continually increasing fields. Thus it seems worth while to endeavor to 

 ascertain whether such a supposition would conform with any reasonable 

 value of the susceptibility k of the iron, which one may estimate as decreasing 

 from over 100 to less than 10, as the rod approaches saturation (#=150 

 gauss) and to decrease thereafter asymptotically to zero. 



If p is the force per square centimeter of section of the rod and E Young's 

 modulus, 



(i) P-E% 



regarding the magnetic stress as traction. 



Using the expression for the potential of a disk, the magnetic force F, per 

 unit pole in a narrow crevasse normal to F, between molecular layers of 

 magnetic surface density a is 



where k is the susceptibility of the metal and H the exciting field without, 

 so that a = kH. Hence the force per cm. 2 should be p' = Fa 



(2) p'= 



Equating p p' in equations (i) and (2) modified, 



(3) A 



If the data in figure 32 are taken above 200 gauss, supposing that these 

 are far enough removed from the initial complications, the estimates would 

 be (E = 2Xio 12 ) 



H io 6 A///, 



(observed) k 

 250 gauss i.o 1.6 



400 3.5 1.9 



800 i i.o 1.6 



