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Proceedings of the Koyal Society of Edinburgh. [Sess. 
to H, and there is no hysteresis. But instability occurs when F reaches 
a value of 149, the deflection then being about 4°’7. At that point (a 
on the curve) the magnets swing violently into a new position of 
equilibrium, for the same value of H, the deflection changing to 16‘’'2 ; 
and if H is then further increased, they are gradually brought into closer 
agreement with the direction of the field, taking that direction only when 
the field is indefinitely increased. Again, if after the field H has been 
raised to a high value, it is reduced, an unstable stage is reached when 0 
is about 12°‘5 and F is 136. The magnets then swing violently back to 
a position at which Q is 2°-4. In the early stage, up to a, and again in 
the final stage from h onwards, the deflection is reversible with varying 
H ; but between the early stage and the final stage there is an irreversible 
step in which the deflection exhibits hysteresis and involves dissipation 
of energy. In the figure the outward and inward irreversible steps are 
exhibited by the lines ah and cd respectively. 
