332 BELL SYSTEM TECHNICAL JOURNAL 



no longer; there are methods for producing large single crystals of 

 metals at will, whether by direct solidification from the melt or by 

 suitable treatment of the masses of randomly-disposed minute crystals 

 which blocks of metals usually are; and there are methods for deter- 

 mining the orientations of the axes of these crystals by means of 

 X-rays. So lately have these methods been developed (they are 

 outgrowths of researches of the last ten or fifteen years) that the first 

 data concerning the ferromagnetic crystals, except for some relating 

 to magnetite and pyrrhotine and a very few early measurements on 

 iron, are only now appearing. One has at times a feeling that these 

 are the first really significant data, the only suitable foundation for a 

 theory of ferromagnetism ; that the properties of a polycrystalline 

 rod or wire or ellipsoid do not form a proper basis for theorizing, not 

 being even a simple average of the properties of single crystals oriented 

 in all directions, but a deformed and distorted average infected by 

 the crowding and the cramping and the squeezing which the little 

 crystals perpetually inflict on one another. 



All but two of the well-known ferromagnetic substances crystallize 

 in the cubic system. (The exceptions are pyrrhotine and one modi- 

 fication of cobalt, which conform to the hexagonal system). In cubic 

 crystals, directions parallel to the edges of the cubes, to their diagonals, 

 to the diagonals of their faces, are called the tetragonal, trigonal, 

 digonal axes, or the quaternary, ternary, binary axes respectively; the 

 planes to which these directions are perpendicular are called (100) 

 planes, (111) planes, (110) planes respectively. This is as much of the 

 technical language of crystal analysis as we shall require. Of the 

 three lattices in which atoms may be arranged in a cubic crystal — 

 simple cubic, body centred, face centred — iron adopts the second, 

 nickel and cobalt the third. The iron-nickel alloys containing more 

 than 30 per cent of nickel copy the nickel lattice (the permalloys belong 

 to this class) while those containing less than 30 per cent of nickel 

 imitate iron. Many other metals which are not ferromagnetic have 

 cubic lattices of the second or third type, none at all a lattice of the 

 first; it is therefore futile to look for any correlation between ferro- 

 magnetism and the arrangement of the atoms. 



When a magnetic field is applied to a crystal, it produces a magnet- 

 ization which is not parallel to the acting field — to the resultant, I 

 mean to say, of the applied field and that due to the "demagnetizing 

 effect of the poles" — unless this resultant is parallel either to a tetra- 

 gonal or to a trigonal or to a digonal axis. If we apply a field parallel 

 to the axis of an ellipsoid or a long rod, cut from a single crystal in 

 such a way that this axis is parallel to one of the specified directions. 



