OF 
Magnetization of Iron in Bulk. 635 
for cast steel containing silicon give an average of 12,000 
of p, but the composition of the steel has so great an influence 
that any numerical estimate is uncertain. If the value chosen 
were too large the roots would also be too large, and the 
amplitudes therefore diminished. The ratio aes is plotted 
in fig. 5; and since the values of H, and H, are both pro- 
portional to the coil-current, this ratio shows the theoretical 
change of the latter with time when the permeability is 
constant. The decay of current in a short-circuited coil is 
in this case the same as the growth of current when first 
switching on, the curve as drawn being merely inverted. 
The curve B of fig. 5 is the rise of current in a coil sur- 
rounding a solid core 41 cms. diameter. The small dip 
observed beyond the bend must be attributed to a rapid 
molecular movement. The change in the curvature is sudden, 
and is always obtained even with repeated magnetizations in 
the same direction. The above analysis shows that no such 
dip would be obtained if the permeability were constant. It 
ean then only be attributed to a sudden increase in the number 
of molecular magnets brought into alignment. The observed 
effect resembles the phenomenon of crystallization in a super- 
saturated solution, starting at the circumference. It is in 
fact the direct observ ation, only possible when dealing with 
large masses of iron, of the sudden slip noticed in the magnets 
of Ewing’s hysteresis model when the field appr oaches 
saturation. 
The growth and subsidence of magnetic induction in 
rectangular cores cannot well be calculated except on the 
assumption that the core currents follow rectangular paths. 
This is approximately true when the thickness is small 
compared with the length of the section, but not when it is 
comparable to it. 
Photographs have been taken by Professor Hele-Shaw of 
the flow of liquid between the minor and major semiaxes of 
rectangles. These show clearly the transition from a circular 
path at the centre to an approximately rectangular path at 
the outer boundary. At intermediate points the curves are 
too complex for the action of the core currents to be 
calculated where the thickness is not small compared with 
the length, on account of the difficulty of finding an expres- 
sion for the instantaneous magnetic force in terms of the 
distance from the centre. 
