.METEORIC AND ARTIFICIAL NICKEL-IRON ALLOYS. 
53 
permeability would not vary appreciably over this range. From subsequent data 
and from reasons given later it seems possible, however, that the permeability may 
become perceptibly less at intermediate temperatures than it was found to be at 
400° C. and 15° C. 
On a later occasion, after thermal treatment, to be described, the permeability 
variation was observed during cooling at a number of temperatures intermediate between 
those just stated. The initial and final permeabilities (see 96 and 100, fig. 13) in 
this case bear practically the same relation to one another as the permeabilities (at 58 
and 59) represented in fig. 10. The 'positions of the points p, q and r were calculated 
on the assumption that the ratio of the permeabilities, at corresponding temperatures 
in the two cases, remained the same throughout the cooling as at 450° C. and 15° C. 
The lack of experimental data that might be of interest was not noticed until after 
the apparatus had been dismantled. 
The general form of the yd curve having been again approximately determined, the 
first experiments after re-winding were made to test, from points in the region where 
the permeability is rising rapidly during cooling, for the reversibility described by 
Guillaume (see above, § 6, p. 50). The experiments were not completely satisfactory 
on account of the difficulty of keeping the temperature absolutely constant for a 
considerable time. In the measurements given under the first winding the second 
value of the permeability at the original temperature of interrupted cooling was 
slightly greater than the first, but as both observations lay in a region where the 
permeability varied rapidly with temperature, it was difficult to decide whether the 
variation was not strictly reversible. In order to make the test less difficult 
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Fig. 11. Meteoric iron (2nd winding). 
Cooling from 850° C. interrupted at 580° C. and at 550° C. (respectively). 
Effect of subsequent reheating and cooling. 
