Page:Scientific Memoirs, Vol. 2 (1841).djvu/30

 18 fluid, or the currents developed by the influence of the currents, reunite as soon as the soft iron is removed. The coercitive force opposes the union of these fluids or currents, and retains them in the position of equilibrium they have assumed. In soft iron, the effect of induction might be considered general; the mass, or rather the entire surface, is equally attacked; in tempered steel the magnetism can fix to such and such spot as is capable of retaining it. It is similar to the electrical condition of isolating substances. The analogy between the figures of Lichtenberg and those of Haldat cannot be misconceived. After this, it would not be astonishing to see the magnetizing of steel, by means of very weak electric forces, under the very conditions which leave soft iron intact, even under the influence of highly energetic forces. Four steel needles, of 0ᵐ·1 in length, and of a thickness of 6ᵐ·001, were fixed symmetrically by their ends to the outer surface of a coil which formed a voltaic circuit conjointly with a magneto-electric helix wound round a horse-shoe. On breaking or perfecting the circuit in $$g$$ or $$h$$ (fig. 6), a magneto-electric current is constantly obtained, sometimes in the voltaic direction, at times in the inverse one. The magnetization of the horse-shoe being very weak, on account of the current dividing itself between the two helices, the magneto-electric current of contact or of disjunction, is also weak. Notwithstanding this, the needles exposed to the shock of a single disjunction were powerfully magnetized, all in the direction which had been previously determined, in conformity with the established law. The following table exhibits the magnetic forces acquired.

It is still necessary to add, that the needles had been purposely arranged so that the poles would be reversed, and this always ensued. Moreover, whilst the voltaic current was actually passing, there was no evident magnetism in the steel needles, as was the case in the soft iron wire.

The law established above (35), and relative to the magnetic condition of the interior of concavities, is neither confirmed nor