Magnetised Iron, $-c. t cooled to Temperature of Liquid Air. 59 



many 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 tempera- 

 ture 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 afc 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 dimin- 

 ished 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 C. 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 tempera- 

 ture 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 



F 2 



