358 Mr. F. 0. Thompson on the Electrical and 



magnetic measurements were made on samples of very pure 

 iron in which, as a result of appropriate heat-treatment, the 

 size of the crystals varied to a marked extent, the material 

 remaining otherwise unchanged. Iron was the metal chosen, 

 since a greater variation of grain-size can be induced in it 

 than is the case with most other metals. An ingot weighing 

 28 lb. was cast from a specially pure Swedish bar-iron, 

 melted by the coke-crucible process. Before casting it 

 0*04 per cent, of aluminium was added to render the metal 

 sound and free from blow-holes. The iron gave the following 

 analysis : — 



Carbon, 0*049 per cent. 



Silicon 0-04 



Manganese 03 ,. 



Sulphur 0-02 



Phosphorus 0*016 ,, 



A trace only of aluminium remained in the ingot, whence 

 the iron percentage by difference was 99*87 per cent. It is 

 therefore one of the purest, if not the purest, ingot of iron 

 ever made by this process. The specific resistance and the 

 magnetic properties were determined in the following five 

 conditions, in each of which the number of crystals per cm, 

 length was carefully determined microscopically: — 



(1) As received from the rolls. 



(2) Normalized, i. e. re-heated to 900° C. in a muffle and 



cooled in air. 



(3) Re-heated to 900° C. and cooled in a muffle during 

 twelve hoars. 



(4) Re-heated to 900° C, maintained at this temperature 

 for fifteen hours, and cooled in the furnace over 

 another forty hours. 



(5) Re-heated to 900° C, quenched in cold water, and 

 tempered at 650° C. (This temperature was cho.sen 

 since it is well above that — 520° C. — at which iron 

 deformed by cold work loses its strain*. Quenching 

 strains which might conceivably vitiate the results 

 are thus removed. Also the small percentage of carbon 

 present will practically revert to the condition in which 

 it is present in the other samples.) 



By these treatments the number of crystals per cm. was 

 varied from 690 in the quenched and tempered sample to 

 about 10 in the drastically annealed one. In the latter case, 

 however, some considerable irregularity in the crystal size — 

 which though not readily explicable, is probably dependent 

 * Goerens, Journ. Iron & Steel Instit. Carnegie, vol, iii. (1911). 



