Page:Proceedings of the Royal Society of London Vol 60.djvu/74

Rh ?nany cases this sudden cooling immediately deprived the magnet of a considerable percentage of its magnetisation, and the magnetic moment was reduced. This, however, was not universally the case. In some cases, as in that of the chromium steels, the first effect of this sudden cooling was an increase in the magnetic moment of the magnet; in other cases hardly any change in the magnetic moment at all. The vessel of liquid air was then removed, and the magnet allowed to heat up again, which it very quickly did, to the temperature of the room, or rather to a temperature at which the deposit of snow formed upon the needle immediately on coming out of the liquid air, fully melted. This was taken to be at about 5° C. It was found that each magnet had certain peculiarities of its own.

Taking first the ordinary carbon steel (a sample of knitting-needle steel) we observe the following facts :—

Knitting-needle Steel (a) Tempered Glass Hard.—The first effect of cooling this magnet was to diminish the magnetic moment by 6 per cent. On allowing the magnet to heat up again to the ordinary temperature, the magnetic moment still further diminished by about 16 per cent. On cooling again the magnetic moment increased 10 per cent., and from and after that time cooling the magnet always increased the magnetic moment, and allowing to heat, up again to ordinary temperature always diminished the magnetic moment, the magnetic moment at —185° C. being about 10 per cent, greater than the magnetic moment at 5° 0. The first effect, therefore, of the cooling was to permanently diminish the magnetic moment, but after a few alternations of heating and cooling, the magnet reached a permanent condition in which its moment, when cold, was greater than its moment when warm. These changes of magnetisation may be best represented as in the diagram in fig. 1, in which the firm lines represent to some arbitrary scale the moment of the magnet when at its ordinary temperature of 5° C., and the dotted lines represent to the same scale the moment of the magnet when cooled to —185° C.

Knitting-needle Steel (b) Medium Temper.—The same general results were obtained with knitting-needle steel tempered to a medium temper. The first effect of the cooling to the low temperature was to diminish the moment of the magnet. On allowing it to heat up again the moment of the magnet diminished still more. The next cooling caused an increase of magnetic moment, and from and after that time the steel settled down into a permanent condition in which the magnetic moment was greater at —185° C. than at 5° C. by nearly 20 per cent, of its value at 5° C. (see fig. 2).

Knitting-needle Steel (c) Annealed Soft.—The same general course of events was noticed in the case of the knitting-needle steel when made soft by heating to a red heat and allowing it to cool very