XXXVII.—On a Sensitive State induced in Magnetic Materials by Thermal Treatment.

SUMMARY1. Nickel and the Heusler alloy give “sensitive states” of nearly 2 and about 5 per cent. respectively for a magnetising field of 8 C.G.S. units.2. Steel wire specimens dropped vertically on a stone slab from a height of 1 metre showed a reduction of 37 per cent, in the “sensitive state” for a single fall, 49 per cent, for three falls, 62 per cent, for ten falls, and 73 per cent, for fifty falls.3. After the “sensitive state” has been removed from a specimen by the process of demagnetising by reversals, it cannot be completely restored by reannealing. That is, the specimens exhibit a fatigue effect.4. In the case of one variety of steel, the “sensitive state” had been reduced to less than one-half its original value after seven annealings, and to one-fifth after seventeen.5. No recovery from the fatigue condition was observed in specimens which had been laid aside for fifty-four days.6. Repeated annealings without intermediate magnetic testing showed neither an augmentation of the “sensitive state” nor a fatigue effect.7. Specimens demagnetised at −190° C., heated to room temperature, and cooled again to −190° C., showed a small “sensitive state” at that temperature.8. Larger effects were induced by heating from −190° C. to 15° C., or by cooling from 15° C. to −190° C.9. A “sensitive state” could be induced by any variation of temperature, but not by exposure to a steady temperature, either high or low. The effect is associated solely with change of temperature.10. The amount of “sensitive state” induced by equal temperature alterations varies with the position of the range on the temperature scale and with the material.11. The change from the “sensitive” to the normal condition is unaccompanied by any appreciable change in the specific electrical resistance or elastic constants of the material.