magnetism by the application of an alternating current, the iron is so altered in magnetic qualities that it is impossible to repeat two sets of observations under precicely similar circumstances. With regard to the unannealed iron, it may be noted that if an ordinary magnetisation curve is taken up to very high magnetisation forces, and the iron then demagnetised by the application of an alternating current gradually reduced, the first magnetisation curve can never be repeated exactly again on applying increasing magnetisation forces, but a curve is obtained which lies slightly inside the first curve, and which indicates that the permeability has been reduced. The subsequent repetition of this process will give a series (of curves which occupy different positions, but which do not precisely repeat any of them. Hence it is impossible to repeat at a constant temperature with this unannealed iron exactly any magnetisation or permeability curve. In the case of the annealed iron it is quite different. A. magnetisation curve can be obtained after having carefully de-magnetised the iron, if this magnetisation is pressed up to nearly its limit and the iron then de-magnetised by the application of an alternating and decaying magnetising force, a second magnetisation curve can be obtained on again applying an ascending magnetising force, but it will not coincide exactly with the first curve. The annealed iron cap, however, be brought back into its original condition by dipping it a few times into liquid air. Under these conditions, we have been able to repeat as frequently as required the observations with the annealed iron taken at the different temperatures. In the case of the unannealed iron the changes produced in it by immersing it in the liquid air and by magnetising and demagnetising it, are such as to render it almost impossible to obtain results capable of precise repetition, with respect to the hysteresis loss and permeability for varying magnetising forces.
Experiments with Hardened Iron.
A third set of experiments were taken with a ring coil of the same dimensions as the ring coil made of soft annealed transformer iron first described. This third coil was constructed of the same sample of Sankey’s transformer sheet iron as the above described soft annealed ring, but it was treated subsequently to its formation in the following manner :—
A short piece of iron gas-pipe was made red hot in a forge; the ring coil, having been constructed, was dropped into the red-hot pipe, and the ends of this pipe loosely plugged up with slag wool; the redhot pipe was then covered over with cinders, and the mass allowed to cool. Under these conditions the ring coil was annealed in an atmosphere of carbonic oxide and in contact with hot carbon ; the sheet
