362 Mr. F. C. Thompson on the Electrical and 



visible to the naked eye after etching. The rest was composed 

 of very much liner grains. The conductivity in this case 

 was the greatest of any of the samples measured, but it was 

 distinctly less than that corresponding to a specimen con- 

 sisting of big crystals alone. The conductivity was, however, 

 greater than w T ould be expected from the relative proportions 

 of the large and small crystals. These regions of tine and 

 coarse crystallization are essentially so many circuits of 

 higher and lower resistance, and it is to be expected that the 

 current will pass preferentially through the coarser crystalline 

 parts of lower resistance. 



Now it has been shown by Benedicks*' that the electrical 

 resistance of steels containing less than 0*5 per cent, carbon 

 varies with the carbon percentage in a different manner from 

 that of steels with a higher carbon content. As a result of 

 much careful work, it was concluded that about 027 per cent, 

 carbon is present in solution in these latter steels giving a con- 

 stituent "ferronite." It would seem, however, possible to 

 explain the altered behaviour of the electrical properties of 

 steel above and below 0'5 per cent, carbon in a simple manner. 

 A low carbon steel consists of a matrix of iron crystals 

 (ferrite) with isolated areas of pearlite in which the carbon 

 present as Fe 3 C is segregated. The electrical resistance of 

 these latter areas is considerably greater than that of the 

 ferrite (the specific resistance of pearlite being about 

 20 microhms per c.c). Hence in such a steel the current 

 will preferentially take a path through the ferrite. As the 

 carbon content of the steel rises the pearlite increases at the 

 expense of the ferrite, and at 0*45 per cent, the volumes o£ 

 each are the same. Beyond this percentage, however, the 

 relative order is reversed, and it is now the ferrite which 

 occurs isolated in a background of pearlite. In this 

 structure the continuous paths of the less resistant con- 

 stituent are wanting; and if is not difficult to realize that 

 the specific resistance now bears a somewhat different 

 relationship to the carbon content from that which holds 

 for those steels in which ferrite is the predominating 

 constituent. Such an explanation removes the necessity 

 for assuming the new constituent "ferronite," The fact, 

 too, that, so far as the author has been able to determine, the 

 pearlite areas in pure carbon steels free from silicon and 

 manganese (which some of Benedicks' steels certainly were 

 not) are directly proportional to the carbon content, is also 



* Benedicks, he. cit. 



