Electrical and Magnetic Properties of Pure Iron. 357 



Similarly, by giving to m other values, we may determine 

 corresponding stresses and plate thicknesses, from which 

 data to construct a curve of "stress with relation to plate 

 thickness/'' as shown in diagram fig. 6. 



If we have previously decided upon the stress to be 

 allowed in the material, we are able by the aid of this 

 curve to determine the necessary thickness of plate re- 

 quired. 



Thus for a stress of 10 tons per square inch (corresponding 

 to a factor of safety of about 3 with mild steel) the thickness 

 of plate required would be '376 inch. 



XL. The Electrical and Magnetic Properties of Pure Iron 

 in relation to the Crystal Size. By F. C. Thompson, 

 M.Met.. J3.Sc, Demonstrator in Metallurgy in the University 

 of Sheffield*. 



DURING- late years very considerable attention has 

 been paid to the crystalline boundaries in metals, 

 and the theory has been advanced, and received consi- 

 derable support, that the metal at the crystal junctions is 

 amorphous, corresponding in many respects to the liquid 

 metal in a drastically undorcooled condition. The mecha- 

 nical effects resulting from such a structure are now fairly 

 well known. The electrical and magnetic properties have 

 not yet, however, received full examination from this point 

 of view. 



So long ago as 1902, Professor W. M. Hicks t, commenting 

 on the high permeability of iron alloyed with aluminium, 

 pointed out that the elements aluminium, silicon, and 

 phosphorus, which increased the permeability of iron, were 

 the elements which had been shown by Arnold J to increase 

 the crystalline size of that metal. " It is probable, therefore, 

 that the increase of permeability due to these substances is a 

 secondary effect due to the size of the crystals." This idea 

 would imply that the magneto-motive force required to send 

 the magnetic flux through the body of the crystal itself is 

 less than that necessary to send it across the glassy material 

 at the crystal boundaries. 



In an investigation on the relationship of the elastic limit 

 of a metal to the size of the crystals §, some electrical and 



* Communicated by the Author. 



t Hicks, 'Nature,' lxv. p. 558 (1902). 



X Arnold, Journ.'lron & Steel Instit. i. p. 107 (1894). 



§ Thompson, Trans. Faraday Society, i. p. 101 (1915). 



