Iron and Steel in a Rotating Magnetic Field. 183 



molecules. By deduction from this theory it lias 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 cir- 

 cuit 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 elec- 

 trical or electro- magnetic nature rather than of africtional 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 sub- 

 jected 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 con- 

 ditions 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 sharply defined maximum and rapidly dimin- 



