238 BELL SYSTEM TECHNICAL JOURNAL 



We see at once that the super-atoms must be very easy to ah^n, 

 because saturation comes so quickly, with so rehitively small a held 

 strength. We learn also that when they are aligned, they are not 

 exposed to the incessant urge to utter dis-alignment which afflicts the 

 atoms of a paramagnetic substance, for iron continues to be magnetized 

 when the field is withdrawn; not fully magnetized, as a rule, but con- 

 siderably so. Heretofore I have been talking of substances, in which 

 the atoms have a natural state of perfect dis-alignment or random 

 orientation ; a moderate field can derange it only a little, and the atoms 

 return to it instantly and invincibly as soon as the field is cancelled. 

 Now I am talking of substances in which the super-atoms seem to have 

 no single natural state at all; a moderate field aligns them with ease, 

 and when it is removed they like to linger in their alignment. The 

 phenomena become clearer when we experiment not with ordinary iron, 

 which is a chaotic mass of tiny crystals, but with a single large crystal. 

 It turns out then that the super-atoms have a mighty preference for 

 pointing along the cubic axes as distinguished from all the other 

 directions; but as between these three cubic axes, and as between the 

 two opposite senses along each of the three, they seem to be well 

 satisfied with any. Suppose for definiteness that I have a cubic 

 crystal of iron with one of its axes vertical, another in the meridian and 

 the third, of course, pointing east and west. Then if the crystal is 

 unmagnetized, one sixth of the super-atoms may be pointing east and 

 one sixth west, one sixth pointing north and one sixth south, one sixth 

 pointing up and one sixth down. (I do not say that this is necessarily 

 the case, but it may be.) Now if I apply to the crystal a moderate 

 magnetic field pointing north, the one sixth of the super-atoms which 

 were already pointing north will not be affected, but all the other five 

 sixths will flop right over and imitate them. It is amazing how small 

 a field will suffice to do this: 100 cersteds for a good single crystal, 

 whereas 100,000 oersteds, as I suggested, are not enough to bring the 

 ordinary paramagnetic substance at room-temperature anywhere near 

 to saturation. If next I cancel the field, the five sixths of the super- 

 atoms which came over to the northward orientation will not be 

 irrestibly urged to hasten back to their previous habit: indeed if I 

 manage to avoid mechanical shocks and jarrings, most of them may 

 linger indefinitely, still pointing in the direction to which the vanished 

 field once tempted them. Some readers may notice an odd resemblance 

 between this and the earlier case, in that the super-atoms have a finite 

 number of discrete orientations, just as the atoms do. This resemb- 

 lance is, however, so superficial and (probably) misleading, that I 

 might not even mention it if I could be sure that it had not been 



