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 prdliug 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., 27rl^ 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 which is of the order 10^ gauss. The localised stress 
across this interface is |-NP or which we have seen to he of the order 2x 10® 
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. NewallI 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 
carefidly 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 O'SxlO® 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 irregidaiities, 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. 
t ‘Phil. Mag.,’ September, 1888. 
