276 MAJOR A. E. OXLEY ON THE INFLUENCE OF MOLECULAR 



conventional dimensions, within an iron crystal where the saturation intensity is 

 I, the mechanical stress is 27rP, which is far smaller than the ultimate tensile strength 

 of the iron. Indeed, EWING* remarks " we may, if we please, regard the magnetic 

 molecules as pulling at one another across any imaginary interface, while the stress 

 with which they pull is balanced by thrust in the framework of the iron, but neither 

 the pull nor the thrust is competent to explain the mechanical strains." The above 

 value of the stress, viz., 27rP is obtained by taking a crevasse whose gap, although 

 " physically small," is sufficiently wide to accommodate several molecules in line. If 

 we take a narrower crevasse, approaching "mathematical smallness" in width of gap, 

 we obtain a measure of the forcive acting between the molecules, and this includes 

 the localised forcive NI or a' c l which is of the order 10 7 gauss. The localised stress 

 across this interface is ^NP or ^a' c P, which we have seen to be of the order 2x 10 9 

 dynes per square centimetre. This is of the same order as the ultimate tensile 

 strength of crystalline media both ferro-magnetic and diamagnetic. 



The magnetic resistance of joints is interesting in connexion with the localised 

 nature of the molecular field in iron. It has been shown by Sir J. J. THOMSON and 

 H. F. NEWALLJ that the susceptibility of an iron bar is much reduced if it is 

 severed and the two parts put in contact. Later, Sir JAMES EWING and W. Low| 

 investigated this effect in a more exhaustive manner when the joints were carefully 

 trued up and also for rough joints, under varying pressures. They found that for a 

 carefully planed joint a compressive stress of 226 kilogrammes per square centimetre 

 restored almost completely the loss of magnetic property produced by cutting, but 

 that this stress had only a small restorative effect in the case of a rough joint. In 

 the latter case, we may suppose that the number of points of contact between the two 

 parts of the bar is small, in the former that the two portions are in contact over a 

 large percentage of the available area of contact. Under a compressive stress of 226 

 kilogrammes per square centimetre, it appears that in the trued-up specimens the order 

 of contact of the molecules is the same as in the uncut metal and therefore this stress 

 is a measure of the internal stress within the material. As 226 kilogrammes per square 

 centimetre is equal to 0'5xl0 9 dynes per square centimetre, this stress, although 

 lower, is comparable with that calculated on p. 252, and we may regard the width of the 

 resulting crevasse as approaching mathematical smallness, the spheres of influence of 

 the molecules on either side of it overlapping to an extent comparable with the over- 

 lap in the interior of the uncut bar (see also Part III., p. 89). But even with the most 

 carefully faced junction there will be irregularities, coarse compared with molecular 

 dimensions, and in such regions the localised nature of the molecular field will 

 determine a finite air gap which would account for the difference of stress mentioned 



above. Perfectly faced surfaces of soft iron or mild steel (annealed) might be 







* ' Magnetic Induction in Iron and other Metals,' p. 254, 

 t ' Proc. Camb. Phil. Soc.,' 1887. 

 | ' Phil. Mag.,' September, 1888. 



