Magnetic Properties of Pure Iron. 359 



on the crystallization which occurs at the critical temperature 

 Ac 3 — rendered it unsuitable for the purpose of directly cor- 

 relating the crystal size Avith the magnetic and electrical 

 properties. The five specimens were cut consecutively from 

 a single length of the rolled bar, between the two ends of 

 which no difference of chemical composition could be detected. 

 After treatment each of the small bars was turned down 

 from the original diameter of one inch to a centimetre. 

 Absolute freedom from alterations in composition due to 

 surface decarburization was thus ensured. 



(a) Electrical Resistance. 

 During the passage of an electric current through a normal 

 metal the opposed resistance may be considered to be the sum 

 of two factors, one for the actual crystalline material, the 

 second depending on the resistance of the amorphous metal 

 at the crystal boundaries. This amorphous metal, which, as 

 already mentioned, is of the nature of an undercooled liquid, 

 may be expected to possess a higher resistance than the 

 crystals themselves, since a substance at its melting-point 

 possesses a higher conductivity in the solid crystalline 

 condition than in a liquid. 



The thickness of the amorphous films is exceedingly small, 

 probably of the order 10 ~ 5 cm.* Thus the length of the 

 crystalline metal to be traversed by the current is essentially 

 unaltered, whatever be the number of crystals per unit length. 

 That part of the observed resistance due to the crystals 

 themselves will, therefbie, be a constant. On the other hand, 

 however, the resistance of the boundaries will be proportional 

 to the number of these regions of decreased conductivity which 

 have to be traversed. 

 Thus, if 



pi is the observed specific resistance, 



p 2 is the specific resistance of the crystalline portion, 



n the number of crystals per cm., and 



R the boundary resistance for each grain, 



then p 1 —-p 2 -rnK (1) 



In order to check this relationship experimentally, bars 

 20 cm. long and 1 cm. in diameter were turned from each 

 of the heat-treated samples. The specific resistance of each 

 was determined by comparing the fall of potential over a 

 definite length (15 cm.) with that in a standard low resistance 

 through which the same current was flowing. The deter- 

 minations were carried out at 16° to 17° C. In Table I. the 



* Thompson, Trans. Faraday Soc. i. (1915)- 



