Page:Proceedings of the Royal Society of London Vol 60.djvu/81

66 marked. Steels having these high percentages of nickel are, as Dr. J. Hopkinson has pointed out,* remarkable for the wide range of temperature within which they can exist in two states, one considerably magnetic, and one practically non-magnefcic or but feebly magnetic. In these two states their mechanical and other physical properties are entirely different. In the experiments here mentioned, the nickel steel samples were iu the magnetic condition. They are put into this condition by dipping for one moment in liquid air, and are only transformed back into the feebly magnetic condition by heating to a cherry-red heat. The 29 per cent, sample of nickel steel being in the magnetic condition was magnetised by contact with the poles of the electromagnet. On cooling it in liquid air it immediately lost about 20 per cent, of its moment, on warming up again to 5° C. it lost about 5 per cent, more, and from and after that point remained in a condition in which cqoling the magnet to —185° C. caused its moment to become about 10 per cent, less than it was at 5° C. Hence the 29 per cent, nickel steel exhibits the same quality but in a less marked degree than the 19 per cent., in that its magnetic moment is decreased by cooling to —185° C., and recovers again on heating up to 5° C. In this respect the two samples of nickel steel differ from all other samples of steel which we have examined, in that they have a negative temperature coefficient for magnetic moment change with temperature, after the first change on cooling has taken place.

Pure Nickel.—In order to see if this peculiarity extended to pure nickel, we examined the behaviour of a small magnet made with Mr. Mond’s pure nickel, but we found that such a nickel magnet, magnetised to saturation, behaved exactly as did a carbon steel magnet (see fig. 15). The effect of the first cooling to the temperature of liquid air was to diminish the magnetic moment. On allowing the magnet to heat up again to the ordinary temperature the moment diminished still more, and from and after that time the behaviour of the magnet was perfectly normal, that is to say, its magnetic moment when at 5° C. was less than its magnetic moment at —186° C., but only by about 3 or 4 per cent, of the latter value. Silicon Steel.—A sample of silicon steel, containing 2'67 per cent, of silicon, behaved in a normal manner (see fig. 16). The magnet experienced a permanent diminution of moment on cooling for the first time, and after that, its magnetic moment when cold was greater than its magnetic moment when warm.

Soft Iron.—In order to determine if similar changes of magnetic moment could be produced in the case of soft annealed iron, small magnets of Swedish iron were prepared, formed of a short length, about 15 mm., of soft iron, or a small slip of annealed transformer iron. On magnetising these in a strong field, and testing them with
 * ‘ Koy. Soc. Proc.,’ 1890, vol. 47, p. 138.