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XVIII. The Hysteresis of Ivon and Steel in a Rotating Magnetic Field. 
By Francis Gibson Baily, M.A., Professor of Electrical Engineering, Heriot- Watt 
College, Edinburgh. 
Communicated by Professor 0. Lodge, F.R.S. 
Received April 9,—Read June 4,—Revised September 4, 1896. 
In a paper on Dynamo Electric Machinery by Dr. John Hopkinson (•' Phil. Trans.,’ 
1896), the suggestion is made that the value of the hysteresis of the iron core of a 
rotating dynamo armature need not be identical with the value obtained when 
the magnetising force is reversed by passing through a zero value. It was sub¬ 
sequently pointed out by Mr. Swinburne that as a necessary deduction from 
Professor Ewing’s molecular theory of magnetism, the hysteresis of iron in a rotating 
field, or of iron rotating in a constant field, should show a distinct diminution in 
value below that in an alternating field, when the magnetic condition of the iron 
approaches saturation. 
According to Ewing’s theory hysteresis is due to the formation of stable magnetic 
combinations between adjacent molecules which tend to resist any movement of the 
molecular magnets caused by change of direction or magnitude in the magnetising 
force. On the breaking up of these combinations by such a change in the magnetising 
force, new arrangements are formed, and the potential energy of position is trans¬ 
formed into kinetic energy of partial rotational movement round the fixed axis of 
the molecule, which is damped out with or without oscillations above the axis of 
rotation. It has been further suggested that the damping process may be due 
to eddy currents induced by the movement of the magnets, but the precise nature 
of these eddv currents, or the extent to which other retarding influences akin to 
mechanical friction or viscosity may act has not been determined. 
In the alternating field there is no continuity of position of the molecules; the 
magnetising force passes through a zero value at each alternation, and the molecular 
combinations may vary in successive alternations, although the average value of the 
induction and hysteresis remains sensibly constant. Increasing magnetising force 
and induction result in increased movement of the molecules with increased 
momentum and consequent dissipation of energy. But in a rotating field of constant 
value there is no diminution of the strength of the magnetising force. The molecules 
are always under the same restraint, and the movements impressed upon them will 
be more uniform and unidirectional. It is true that with small magnetising forces, 
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