SECTIONAL TRANSACTIONS.— A. 385 



are those of accounting for the number of effective spins per atom, and the 

 magnitude of the interchange interaction in different materials, and of 

 developing a quantitatively satisfactory treatment of the temperature 

 variation of magnetisation and related effects. 



Of most importance technologically is the behaviour of ferromagnetics 

 in relatively low fields. The determinative factors for the sequence of 

 reversible and irreversible processes occurring during magnetisation include 

 the natural crystal anisotropy, the magnetostrictive properties of the material, 

 and the distribution and magnitude of internal strains. Although a quali- 

 tative interpretation can be given of the main effects, the development of a 

 quantitative treatment for particular materials is as yet at an early stage. 



Dr. A. J. Bradley. — X-ray structure and ferromagnetism (12.10), 



Ferromagnetic alloys contain iron, cobalt or nickel (in the Heusler alloys, 

 manganese). They have crystal structures of a simple type : — face-centred 

 cubic, body-centred cubic and (rarely) hexagonal close-packed. Of these 

 types the body-centred cubic is the most favourable for the development 

 of magnetic properties. Some non-magnetic alloys become magnetic 

 when the structure is changed to body-centred cubic from another form. 

 The materials used for permanent magnets (known as ' hard ' magnetic 

 materials) must be distinguished from ' soft ' magnetic materials such as 

 are used for transformer cores. The latter have well-formed crystals, the 

 structures of which are in no way abnormal. The former, though 

 essentially of the same types (body-centred cubic and face-centred cubic), 

 never have perfectly formed crystals. Some kind of strain is essential for 

 the development of high coercive force. For example the alloy may be on 

 the point of breaking up into two phases of different compositions. The 

 mechanism of this process may be such as to produce an intermediate 

 metastable state. The alloy remains a permanent magnet so long as this 

 state persists. Careful heat treatment is required to ensure that decom- 

 position proceeds only to the point where the alloy has the best magnetic 

 properties. 



(Continued below). 



Afternoon. 

 Symposium on Nuclear physics (continued from Thursday) (2.15). 



Prof. R. Peierls. — Resonance in high energy reactions. 



Mr. S. Devons. — Resonance scattering of a. particles . 



Prof. C. D. Ellis, F.R.S. — Resonance levels in slozv neutron processes. 



Dr. P. B. Moon.— ^ slow neutron velocity spectrometer. 



Prof. E. J. Williams. — Loss of energy by fast particles in nuclear 

 collisions. 



Dr. W. E. BuRCHAM. — Disintegration of fluorine by protons and deu- 

 terons. 



Dr. M. GoLDHABER. — Radioactivity produced by nuclear excitation. 



