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



results are collected, together with those calculated by means 

 of equation (1). In every case an increase in the crystal 

 size is accompanied by a corresponding decrease of the 

 specific resistance. To about 1 per cent., which is quite as 

 near^ as the mean crystal size can be determined micro- 

 scopically, the relationship given above is found to hold 

 good. 



Table I. 

 Specific Resistances of Pure Iron. 



Treatment. 



No. of Crystals 

 per cm. 



Specific Resistance in microhms 

 per c.c. 



Observed. 



Calculated. 



Difference. 



As received 



Normalized 



426 

 343 

 276 



? 



690 



7-563 

 7-513 

 7-251 

 7-152 



7-986 



7-564 

 7-422 

 7-307 



8019 



-•001 

 + •091 

 -•056 



-033 



Annealed 



Drastically annealed 



Quenched and tempered... 



The values experimentally obtained render it possible to 

 calculate the true specific resistance of the crystalline material 

 free from junctions, and substituting the mean values thus 

 found in equation (1), the specific resistance of any sample 

 of pure iron becomes 



p = 6*83-f l*72nx 10 3 microhm per c.cm. 



(2) 



The value 6'83, the only rational one to adopt for the 

 constant for pure iron, is a value much less than that usually 

 adopted. In view of this result, no determination of the 

 specific resistance of any metal is complete which does not 

 give the crystal size and also sufficient data for the calcu- 

 lation of p 2 . The wide divergence of the specific resistances 

 (7*15 to 7*99) seems to explain the great variation in the 

 values obtained for such " constants y> by different investi- 

 gators, even when pure materials have been employed. The 

 variation observed in the case of iron is probably greater 

 than in that of most other metals, for instance copper, since 

 in these latter cases the crystalline structure is always coarse 

 compared with the normal crystallization of iron. Hence 

 the factor n is subject to smaller variations, and different 

 determinations show less discordance. One case in which 



