Page:Proceedings of the Royal Society of London Vol 60.djvu/85

70 magnetism. On cooling it to —185° C., it slightly increased in magnetic moment, and on heating np again to 5° C., its magnetic moment decreased to the extent of about 3 per cent, of its moment at —185°C. There was no initial decrease of moment in this case. In this respect, therefore, it resembled the chromium steel magnets. Broadly speaking, the results so far obtained are:—

(1) That the sudden cooling to the temperature of liquid air usually permanently decreases the magnetic moment of short magnets made of many varieties of steel, assuming them to have been initially magnetised in a strong field.

(2) This initial decrease is. found both in hardened steels bavins' great coercive force, and also in the same steels in a soft or annealed condition, and is especially conspicuous in the case of the 19 percent, nickel steel.

(3) In the case of most steels so far examined, the effect of cooling magnets made of them to —185° C. is to temporarily increase the magnetic moment after the permanent magnetic condition has been reached.

(4) The exceptions to the above rule so far noted are the nickel steels with percentages of nickel from 19 to 29 per cent., in which case the magnetic moment is always decreased temporarily by cooling to —185° C., after the permanent magnetic condition has been reached.

(5) It appears from these experiments that one of the best ways of ageing a permanent magnet is to dip it several times into liquid air. It then arrives at a constant condition in which subsequent temperature changes have a definite effect, and in which the subpermanent magnetism is removed.

Since the 19 per cent, nickel steel magnet increases in magnetic moment when heated from —185° C. to +o°C., and since it is well known that at some higher temperature it would lose magnetic moment altogether, it was considered very desirable to ascertain the temperature at which it would have its maximum magnetic moment. The magnet was accordingly heated (on April 2) in an oil bath gradually up to a temperature of about 300° C., and the deflections of the magnetometer observed at intervals, both as the temperature rose and as it fell. The result showed that this nickel steel magnet continued to increase in magnetic moment, until a temperature of about 30J 0. was reached, and the magnetic moment then began to decrease.

At a temperature of +300° C., the moment of the magnet was not much greater than it was at —185° C. On cooling down again from