The dipping-needle is a magnetic needle, the opposite poke of which possess equal magnetic intensities. It is attached to the centre of a vertical circle, and its motion is confined to the plane of this circle. The circle has a motion in azimuth about a vertical axis, and within a fixed horizontal circle, both circles being graduated. When the vertical circle is turned round its axis until the needle acquires a vertical position, the plane of the circle is then perpendicular to that of the magnetic meridian, and hence by meana of the horizontal circle the position of this meridian plane becomes known. The vertical circle with the needle is now brought to coincide with the meridian plane, and the angular depression of the north pole of the horizon, or more strictly of the magnetic axis, may be read off the graduated limb of the veitical circle, and measures the dip. The right fine joining the north and south poles of the needle is nearly coincident with the magnetic axis, but the latter may be ascertained more accurately by inverting the needle and taking the mean direction between its two positions of equilibrium. There are other methods of adjusting the dipping needle, but in every method it requirea great delicacy and minute attention to all parts of the adjustment.
If we place a bar of soft iron, suspended by a collection of silk strings at its middle, in a direction parallel to the magnetic axis of the dipping-needle, the action of terrestrial magnetism will have full effect on the bar, its natural magnetisms will be decomposed, and it will acquire a polarity similar to that of the needle, its poles repelling the similar poles of the needle, and attracting the contrary poles. Its want of coercive power prevents it from retaining the polarity of its different parts when tho bar is moved into other positions; for if we invert the position of the bar, that point which was primitively the north polo will now become the south, and rice rend, under the effects of a new decomposition of its magnetisms by terrestrial influence. If however the bar be left for a long time in the direction of the magnetic axis, or if it be heated to a red heat and suddenly cooled by immersion in water, it will acquire a coercive force, and become permanently mag netic. Iron crosses, weathercocks, &c., which have been long kept in a fixed position, or have been struck by lightning, acquire magnetic properties in te manner above described.
It Is a remarkable circumstance connected with the change of mole cular disposition caused by the action of heat, that if we gradually heat a bar of iron, the intensity of its action on a magnet increases, and arrives at a maximum when the bar is brought to 21 cherry-red heat with higher degrees of heat the intensity is diminished, and is totally inappreciable when the bar has reached a bright white heat ; on cooling it recovers its powers of action by similar steps, and the same law holds true if tho magnet be heated instead of the bar. Hence in producing the greatest development of magnetism by influence, we see the advan tage of using iron or steel bars at a red heat. [LIGHTNING.] Magnetism may be developed in iron, steel, cobalt, and nickel, by other means than the influence of bodies already magnetised, as twisting, hammering, electrical discharges, and galvanic currents. [ELECTRO
MAGNETISM.] If we place a bar of iron in a vertical position, and give it a series of slight blows with a hammer or poker, it will acquire a feeble degree of magnetism ; hence it happens that the anvils and other tools employed in smithies are endowed with magnetism. In all such cases the mechanical operations tend to bestow a coercive power, while the terrestrial magnetism separates the fluids in the body.
Cavallo, Benett, and Coulomb remarked the indications of magnetism given by various substances, as copper, silver, &c. It is particularly observable in hammered copper, and scarcely perceptible when the copper has been cast. Coulomb formed very fine needles of various substances, and suspending them by silk strings between the opposite poles of two powerful loadstones, found that they were acted on by the latter. This phenomenon was attributed to the existence of minute quantities of iron, or iron compounds in those different bodies, and the intensity of the magnetic action Coulomb found from direct experi ments to be proportional to the quantities of iron contained in the bodies, and he afterwards applied this principle to discover the quantity of iron contained in impure metals. Faraday has however shown in the discovery of diamagnetism, that a much deeper truth was involved in the subject, as will be noticed more fully presently.
From the preceding observations on the properties of the magnetic fluids it will be easy to understand the principles upon which the various modes of constructing artificial magnets are founded, which we shall now briefly notice. The earliest method of magnetising a bar of bard iron or steel was by drawing it throughout its whole extent at right angles over one of the poles of a strong magnet. In this case if we suppose that pole which contains the austral fluid to be used, the first contact with the bar decomposes its neutral magnetisms, attracting to the point of contact the boreal and repelling the austral ; the successive parts of the bar are subject to a similar decomposition of their fluids, but it is evident that the effect of each previous decom position neutralises the succeeding, except at the extremities ; the magnetism thus developed is therefore feeble, and apparent only at the extremities of the bars, or in some consecutive points formed by peculiarities in the material of the bar, or in the mode of operation. Dr. G. Knight greatly improved the mode of magnetising bars in the following manner : he joined two strongly magnetised bars by their ends bearing contrary names, and placing on them in the direction of their length a small steel bar heated to a cherry-red heat, with its middle on the point of junction of the magnetic bars, he made each of them to rub on the corresponding extremity of this steel bar, and the latter when removed was found to be strongly magnetised. In this method not only does the presence of the second magnet favour the decomposition of the magnetic fluids, hut the intensity of the action of the magnetic forces is greatly increased by the elevated temperature of the steel bar.