Mordey effect

The effect arises from a difference between two physical situations in which iron cores experience cyclic magnetisation. In a rotating armature, the core moves continuously through a constant magnetic field, so its magnetisation is carried through a cycle by rotation. In a transformer core, the magnetising force periodically reverses direction, passing through zero with each cycle. Mordey observed that hysteresis loss — the energy dissipated as heat during each cycle of magnetisation — is measurably smaller in the rotating case than in the reversing case.^[1]

Silvanus P. Thompson expressed the effect as follows: "When an armature core is rotated in a strong magnetic field, the magnetization of the iron is being continually carried through a cycle, but in a manner quite different from that in which it is carried when the magnetizing force is periodically reversed, as in the core of a transformer. Mordey has found the losses by hysteresis to be somewhat smaller in the former case than in the latter."^[1]

Mordey also published related observations on iron permeability in "Slow Changes in the Permeability of Iron," presented to the Royal Society (*Proceedings of the Royal Society*, vol. 57, p. 224).^[2]

Mordey's observation contributed to a broader Victorian investigation into the behaviour of iron under rotating magnetic fields. John Hopkinson suggested in an 1896 paper on dynamo electric machinery that hysteresis in a rotating armature core need not be identical to that measured under an alternating field.^[3] James Swinburne subsequently argued from Professor Ewing's molecular theory of magnetism that hysteresis in iron rotating in a constant field should show a distinct reduction below that in an alternating field, particularly when the magnetic condition of the iron approaches saturation.^[3]

These theoretical predictions were confirmed experimentally. Francis Gibson Baily published measurements of hysteresis in iron and steel under rotating magnetic fields in the *Proceedings of the Royal Society* in 1897, demonstrating that hysteresis loss under a constant rotary field is indeed lower than under an alternating field.^[3] Experiments by R. Beattie and R. C. Clinker in 1896, published in *The Electrician*, addressed the same question at moderate flux densities of the kind used in transformer work.^[2]

The physical explanation, consistent with Ewing's molecular theory, is that in a rotating field the molecular magnets of the iron are carried round continuously rather than being repeatedly reversed through zero. A process that requires less energy to overcome the stable magnetic combinations between adjacent molecules that give rise to hysteresis.^[3]

The Mordey effect is an early instance of the distinction between rotational and alternating hysteresis losses. A distinction that remains relevant in the analysis of core losses in modern AC rotating machines, where rotational magnetic fields exist in the T-joints of transformer cores and behind the teeth of AC machines.^[3]