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BELL SYSTEM TECHNICAL JOURNAL 



around the wire from top to bottom, like the frieze of the Vendome 

 Column; the expansion (or contraction) of the material along this 

 line of force requires the wire to twist. 

 + 20 



^ K—T---. ; 



^x,0* 



-20 



-40 



ZOO 



400 



feOO 



800 



1000 



Fig. 4 — Magnetostriction (Joule effect) in polycrystalline wires of iron, cobalt 

 ("cast" and "annealed"), and nickel. (After K. Honda and S. Shimizu.) 



It is customary to say that, in a gradually-increasing longitudinal 

 magnetic field, nickel contracts continually; cobalt contracts at first, 

 then returns to its original length, then expands; iron first expands, 

 then returns to its original length, then contracts; the Heusler alloys 

 expand continually. Unfortunately, some at least of these statements 

 are valid only for samples which have been and are being treated in 

 particular ways. One finds in the literature, for instance, the infor- 

 mation that hard steel and very -well-annealed cobalt behave like nickel, 

 shortening continually as the field is augmented. If the rules which 

 I stated at first are really typical of the respective elements in standard 

 states, then one may lay what emphasis he chooses on the fact that 

 the four consecutive elements which are nickel, cobalt, iron and the 

 manganese which is the essential element of the Heusler alloys are 

 associated each with a different one of the four conceivable permuta- 

 tions of expansion and contraction. 



The change in length, whichever its eventual sign, comes to an end 

 when the material is magnetized to saturation. Intensity of magnet- 

 ization is therefore the natural independent variable on which to 

 consider magnetostriction as depending. 



Quite the most exciting of the lately-discovered facts about magneto- 

 striction is disclosed in Figure 4a, which consists of curves representing 



