ON THE MEASUREMENT OF MAGNETIC HYSTERESIS. 
87 
The curves tend to confirm the result of § 61, viz., that for small amounts of 
permanent set both Bq and W increase when the couple is removed. The sixth 
curve shows that when the permanent set is large the removal of the couple causes a 
decrease of both and W. In these experiments the change from increase to 
decrease occurs somewhere between 25 and 50 revolutions, as against about 13 in 
the experiments of § 61. The wire of § 61 was not, however, subjected to cycles of 
twisting. 
The curves for zero permanent set correspond to the curves for Hq = 5 found in 
§58 and shown in fig. 11. But instead of curves for only half a cycle of twisting we 
have now curves for a complete cycle. The mean values of Bq and W for zero 
couples are very nearly the same in the two cases, but the couple due to a load of 
300 grammes produces a decrease of Bq and W rather greater than was found in the 
experiment of § 58 for Hq = 5. The specimen had, however, been jjut througli many 
cycles of twisting in the interval. 
The observations could not be extended beyond 50 turns of permanent set, for 
when the attempt v^as made the wire broke at 54 turns. .The similar wires used in 
the experiments of §§ 60, 61 did not break till 104 and 101 revolutions respectively. 
It is possible that the difference, if not due to a flaw, is due to the effect of the cycles 
of positive and negative couples. 
The curves of figs. 13, 14 show in a very striking manner the close correspondence 
between W and Bq when Hq is kept constant and Bq is made to change by varying 
the stress. 
Development of a Cyclic State after Initial Permanent Set. 
§ 63. In the last series of experiments made in 1890 we studied the manner in 
which a wire, after torsional overstrain, gradually settles down to a definite cyclic 
state as the applied couple goes through a series of cycles. The sj)ecimen was a 2 :)iec.e 
of the same annealed soft iron wire as that used in §§ 58, 60, 61, and had the same 
length as the wire used in § 60, viz., 65'5 centims. In these experiments the wire 
was strained far beyond the elastic limit by turning the torsion wheel through a 
definite number of revolutions—in our experiments 6, 20, and 50—from the position 
for zero strain. The wheel Ijeing now clamped, a weight of 300 grammes was hung 
from the right edge of the wheel, producing a couple having the same sense as that 
which strained the wire. The wheel was then undamped, and the Avire, being 
alloAved to untwist, turned the Avheel back until the couple due to the elasticity of 
the wire balanced the couple due to the 300 grammes. After 20 cycles of magnetisa¬ 
tion, H varying between the limits di 5, Bq and W were determined. The load was 
then reduced to 200 grammes, and the magnetic tests were repeated. When by two 
more steps of 100 grammes the load had been reduced to zero, loads of 100, 200, and 
300 grammes were hung in succession from the left edge of the wheel to produce 
