molecules. By deduction from this theory it has been surmised that the hysteresis in magnetic metals under the influence of a constant rotary magnetic field will be less than that in an alternating field in which the magnetising force passes through a zero value. As familiar practical examples of the two conditions may be instanced : the armature core of a continuous current dynamo, and the iron circuit of an alternating current transformer or choking coil.
It is supposed that residual magnetism is due to the combination of molecular magnets in stable magnetic arrangements, and that the energy dissipated in any magnetic change corresponds to the work done in breaking up these arrangements. This energy is rendered kinetic by the movement of the magnets to form new combinations, the magnets either oscillating about the new position or moving to it aperiodically, according to the amount of damping to which they are subject. It is further suggested that the damping is of an electrical or electro-magnetic nature rather than of a frictional character, being produced by the effect of rapid oscillations of the magnets on the surrounding particles or medium. Hence any movement of the molecular magnets during which the formation of new combinations is checked or prevented will take place with considerable reduction in the energy loss due to this cause.
Such a condition is realised when the magnetic substance is subjected to a rotary magnetic field of sufficient strength to force the molecules to maintain a direction parallel to that of the field. If hysteresis is due only to the formation of new combinations and not to mechanical restraint, then under these conditions it will vanish altogether.
Experiments were carried out to verify this deduction. A finely laminated cylinder of iron was suspended on its axis between the oles of an electro-magnet which was capable of rotation about the axis of suspension of the cylinder, thus producing a magnetic field rotating in a plane at right angles to this axis. The cylinder, though otherwise free to rotate, was restrained from continuous rotation by a spring, and the angle of rotation and consequent restoring force of the spring was indicated by a beam of light reflected from a mirror on the cylinder. The speed of the electro-magnet and the exciting current could each be varied.
On rotating the magnet, the armature was dragged round until the restoring force of the spring equalled the force due to hysteresis, and the value of the latter could be obtained from the observed deflexions. The result showed that the value of the hysteresis under these conditions was very different from that obtained in an alternating field. At first the value was higher for corresponding inductions, but at an induction of about 16,000 in soft iron and 15,000 in hard steel the hysteresis reached a shai'ply defined maximum and rapidly dimin-
