ON THE MEASUREMENT OF MAGNETIC HYSTERESIS. 
83 
revolutions, measured from its position when the wire was initially free from torsion 
the number being read on the “ counter.” The wheel was then clamped in this 
position, and, after 20 cycles of magnetisation, observations were made for and 
W. The wheel was now undamped, and the wire was allowed to untwist so as to rid 
Fig. 12. 
itself of torsional stress ; after twenty more cycles of magnetisation, the new values 
of Bq and W were then determined for this condition of the wire. The wheel was 
then turned still further, and fresh ohservatious were made, the jDi’Ocess being 
repeated until the wire broke. The results are shown in fig. 12. The curves for Bq 
and W when the wire was under torsional stress are marked (2a), and those for zero 
stress are marked (26). For the sake of clearness, the origin for these four curves is 
at the right side of the diagram. 
The wire in untwisting turned the wheel back through an angle depending upon 
the twist from which it was endeavouring to rid itself—an angle which increased with 
that twist. After a twist of one revolution the wire turned the wheel back through 
100°, retaining a “permanent set” of 26.0°, wliile, after a twist of 100 revolutions, 
the wheel was turned back through 290°. In the curves (26) the points for Bq and W, 
M 2 
