INFLUENCE OF MECHANICAL ACTIONS ON MAGNETISATION. 643 



M. Berson observed, in this way, curious effects of temperature. 

 With tempered steel, which has been reheated to a higher tem- 

 perature than will be used in the experiments, the temporary and 

 the total magnetisation increase with the temperature between 

 and 340, while the residual magnetism steadily decreases. The 

 variations in temperature during the action of the magnetising force 

 have a marked influence. For a bar magnetised at 290, for instance, 

 the three kinds of magnetisation temporary, residual, and total 

 are considerably smaller than if the magnetising force has been 

 applied while the temperature rises from 240 to 290. 



Singular cases of unstable equilibrium are met with. A steel 

 bar, tempered and not reheated, magnetised at 240, and suddenly 

 cooled immediately after magnetisation, retains a residual mag- 

 netisation considerably higher than that which would be acquired 

 in the cold, under the influence of the same magnetising force. 

 But the bar is, so to say, supersaturated ; for this magnetisation 

 disappears under the influence of shocks and agitations far more 

 rapidly than that which would be produced in ordinary con- 

 ditions. 



1210. MECHANICAL ACTIONS. This influence of magnetic 

 actions on magnetisation is a general fact, and one long known. 

 If a bar of soft iron be placed parallel to the terrestrial field, the 

 temporary magnetism which it acquires in this situation becomes 

 permanent when the bar is struck at one end by a hammer. 

 When struck again transversely to the field, all the magnetism is 

 destroyed. Besides the influence on the coercive force, we may 

 say that any shock or vibration aids the effect of the actual 

 magnetising forces as if they facilitated the arrangement of the 

 molecules by making them more mobile. Thus during mag- 

 netisation the shocks contribute to increase the magnetic moment 

 of the bar. After magnetisation, shocks promote demagnetising 

 actions, and may, as we have seen, cause a steel bar to lose 

 great part of its magnetism. 



Joule* has proved that an iron bar lengthens during mag- 

 netisation; when the action of the field is suppressed the bar 

 shortens, without however reverting to its original length. The 

 elongation produced by magnetisation is not so great if the 

 wire is strongly stretched ; the wire shortens even, if it is 

 subject to a tension near that which produces a rupture. Cobalt 



* JOULE. Annals of Electricity, Vol. vin., p. 219. 1842. Scientific Papers, 

 pp. 46, 205. Phil. Mag. [3], Vol. xxx., pp. 76, 225. 1847. 



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