and Electrical Resistance of Iron. 229 



strengths of magnetic field are shown in the curves 11, 12, 

 and 13. 



A reference to these will show that the difference between 

 annealing at these two temperatures is very marked ; the 

 iron having by the last annealing been rendered much more 

 susceptible to weak magnetic fields. Annealed at 840°, the 

 maximum permeability at ordinary temperatures was about 

 4000 in a field of 1-84 O.G.S. units; annealed at 1150° this 

 rose to the remarkably high value of 46*80 in the much 

 smaller field of 1*48. The effect of this difference in anneal- 

 ing is, at temperatures immediately below the critical point, 

 even more striking, as is shown by a comparison of the yt,-t 

 curves (Nos. 11 & 12) for values of H below unity. 



An interesting point with regard to specimen B is the 

 diminution of permeability which it experiences in the neigli- 

 hourhood of 550° C. This diminution can be traced on the 

 fi-t curves for all but the highest values of H, though it is 

 especially noticeable when the magnetizing force is about 

 1 C.G.S. unit. The effect is more pronounced after the 

 annealing at 1150° than after that at 840°. This peculiarity 

 is hardly, if at all, possessed by specimen A, though it must 

 be borne in mind that the temperature-interval between ob- 

 servations is here much greater, and the effect may thus be 

 disguised. In some earlier experiments made on a ring- 

 magnet having soft commercial iron wire for its core, the 

 minimum of permeability, which occurred at about 500°, was 

 still more marked than in specimen B ; the depression being 

 noticeable in the fi-t curves for fields as large as 5 C.G.S. 

 units. It may be mentioned that, in a list of those tempe- 

 ratures at which the physical properties of iron undergo 

 marked alteration, Tomlinson* gives 550° as that at which 

 the internal friction of soft iron begins to rapidly increase, 

 and at which the " specific heat of electricity^ for that material 

 changes sign. 



The permeability tinder small magnetizing forces (H < '5 

 C.G.S. unit) rises at first slowly ( - 2 to *3 per cent, per degree) 

 when the temperature is raised ; then, in the neighbourhood 

 of 300°, quite rapidly. It remains nearly constant between 

 400° and 550°, after which it rises with increasing rapidity 

 to an enormously high value (//.= 12660 for H = -078 is the 

 highest observed) only to sink still more rapidly at the critical 

 temperature to a value quite insignificant. 



As the magnetic field gets smaller (curves 11 and 12) the 



* H. Tomlinson, Phil. Mag. vol. xxvi. July 1888, p. 21. 

 Phil Mag. S. 5. Vol. 44. No. 268. Sept. 1897. S 



