the Magnetic Permeability of Nickel and Iron. 198 
(1) For a high temperature, the maximum permeability in 
any one curve occurs at a smaller value of the field than for 
a lower temperature. 
(2) The increase in susceptibility under small fields as the 
temperature is raised is notable, although it is relatively 
much less than in the case of iron. 
(3) At temperatures which approach the critical point 
there is more susceptibility to weak than to strong fields, 
while at lower temperatures the reverse is the case. 
Figs. 5 and 6 show how the permeability varies with tempe- 
rature for different values of the field. Curves I. and II. 
are derived from observations under fields of 10 0.G.S. and 
40 C.G.S. respectively (Table IV., p. 203). 
For temperatures below 180° the permeability under a 
10 C.G.S. field is less than that under a field of 40 units. Up 
to 300° the permeability under a high field is fairly constant, 
but above that temperature it begins to fall off, until at 374° 
it is practically reduced to unity. 
For the lower field, ~ reaches a maximum of about 180 at 
a temperature of 320°, after which it falls off more suddenly 
than in the previous case, reaching a minimum value at the 
same temperature as in curve I. 
The critical temperature (374°), at which the magnetism 
of this specimen of nickel practically vanishes, is independent 
of the strength of the field to which the metal was subjected. 
The curves in fig. 6 show wT curves for a specimen which 
was annealed at a slightly higher temperature. 
The hysteresis loops shown in fig. 9 were obtained under 
magnetizing torces of +45 C.G.S. units. They are plotted 
from values given in Table III. (pp. 200-202). In fig. 10 are 
shown cutyes representing the change in “‘ Coercive force” and 
“Residual magnetism,” with temperatures. Both quantities 
gradually decrease as the temperature rises, and they both 
vanish at the “ Critical temperature” 374°. The values are 
plotted from Table V. (p. 203) which was compiled from the 
complete cycles of Table II., Set II. 
Fig. 11 (p. 194) and Table V. represent the temperature 
variation of the hysteresis loss in ergs per cubic cm. per cycle, 
calculated from Set LI. 
This curve also cuts the axis of temperature at a point a 
little below the critical point, the hysteresis undergoing a 
rather more rapid reduction above 250° C. Up to this tem- 
perature the curve is a straight line. 
