Page:Proceedings of the Royal Society of London Vol 60.djvu/83

68 F ig. 16.—Silicon steel. Si = 2-67. C =0-20 Mn = 0-25 Fe = 96 -88.

into a U sliape, with ends about 10 mm. apart, and in this case the changes of magnetic moment, as shown in fig. 17, wei’e similar to

Fio. 17.—Soft iron.

those in the case of the carbon steels, only with very much narrower limits of variation. The first cooling hardly produced any change whatever in the magnetic moment of the magnet. On allowing it to heat up again, the magnetic moment was very slightly diminished, and thenceforth the changes of moment were such that the magnetic moment was greater when the magnet was cold than when it was warm, by about 2 or 3 per cent, of the latter value. In the case of a straight, soft iron magnet, formed of annealed transformer iron, the curious fact was noticed that whereas a rapid cooling of the magnet by plunging into liquid air hardly produced any effect on the magnetic moment after the first initial loss of magnetism had taken place on cooling, the effect of a slow cooling down to the temperature of —185° C. was always to produce a permanent diminution of magnetic moment. Hence the magnetism of this soft iron sample could be frittered away by a process of slow cooling to ~185°C., and intermediate heating up to 5°C. These changes of moment are represented in the diagram of fig. 18.

Hard Iron.—A sample of the same iron, hardened by hammering, was tested, and was found to behave in a very similar manner to the