Du Hamel placed two steel bars of equal length parallel to each other, connecting their corresponding extremities by pieces of soft iron interposed ; then taking two bundles of magnetic bars, he united their poles of contrary name near the middle of one of the steel bare, and by inclining the bundles made one of them pass towards one extremity of the other bar, the second passing in the contrary direction, and then successively repeated the operation, when both the steel bars became strongly magnetised, but with contrary magnetisms at the corresponding extremities of each. In this method the decomposition of the neutral magnetisms of the interposed pieces of soft iron adds to the effect produced by the contact of the magnetised bundles with the steel bars.
Epinus, adopting a similar method, preferred interposing strong magnets instead of soft iron, the relative position of the poles of the two magnets being reversed ; Coulomb combined the advantages of these different methods by composing his magnetised bundles of bars at a cherry-red heat. A fine steel needle may be very strongly magnetised by being placed in the axis of a wire twisted into the form of a helix, the extremities of which are brought in contact with the wires of a powerful galvanic battery. The poles of a bar magnetised to saturation are near its extremities, within generally a few lines, while the intensity becomes insensible at the distance of a few inches ; in a thin bar the intensity may be represented by the difference of the ordinates of two logarithmic curves; the origin of one being at the austral, and of the others at the boreal extremity of the needle.
When bodies containing neutral magnetisms are made to rotate rapidly round an axis, the magnetism becomes developed and acts on the needle ; thus a plate of copper made to revolve rapidly in a horizontal plane will influence- a compass-needle placed over it, and produce in it a rotation in the same direction, on which subject several valuable observations have been made by M. Arago, Sir John Herschel, and others. It has also produced a second mathematical memoir from M. Poisson, in which the mechanical force generated by rotation is introduced into the general equations deduced from his theory of the distribution of magnetism in bodies. The phenomena are however now absorbed into .-MAGNETO-ELECTRICITY.
The consideration of the distribution of magnetism throughout the globe has led to various explanatory hypotheses since the time of Halley ; the position, the number, and the motions of the points which may be regarded as poles of terrestrial magnetism, have been all subjects of discussion and of opinions formed on inconclusive grounds. But the consideration of this subject as well as an account of the dip, variation, and intensity, at different parts of the earth, as well as the consideration of the magnetic equator and poles, and the magnetic observatories that have been erected in different places will be best considered Glider TERRESTRIAL MAGNETISM.
Some important discoveries in magnetism have been made of late years, and some knowledge has been gained as to the molecular changes produced by the magnetic condition. Mr. Joule has shown that a bar of iron in being magnetised increases in length, while its breadth diminishes. The greatest elongation observed by him amounted to the 180-thousandth of the length of the bar. He also states, that the elongation is proportional to the square of the developed magnetic intensity. He has also found that iron wires of a certain tension diminish in length instead of increasing, and that at a certain tension, there is no alteration in length. The sounds produced in making an electro-magnet are noticed under ELECTRO-MAONETISM. Cir cular magnetic polarisation is a new and interesting branch of physical science, which we owe to the genius of Faraday. A few words on the subject will suffice here :—if in the course of a ray of light polarised by reflection, a bar of heavy glass (silico-borate of lead) be placed, it will be seen by a Nicol's prism that the ray is still polarised, and in the same plane as before. If, however, the poles of a powerful electro-magnet be brought near the bar of glass, in such a position that the direction of the ray through the glass is parallel to the straight line which joins the poles, a remarkable effect will be produced on making the electro-magnet. Supposing the Nicol's prism to be in the position of complete ex tinction, so that no light is transmitted through it, and the voltaic current be then sent through the coils of au electro-magnet, the light will immediately reappear through the prism, and will continue as long as the electromagnetic force be maintained, and no longer. [PO LARISATION]. In the course of these researches, Faraday noticed that when the bar of heavy glass was suspended between the poles of the electro-magnet, no sooner was the magnet made, by connecting the coils with the battery, than the bar of glass ceased to swing in differently, but moved round and took up a position at right angles to the direction that would be taken by a bar of soft iron placed in the same part of the magnetic field. The bar made a few vibrations round this position, and finally settled in it ; and when displaced, it returned to it, and settled as before in this position, which Faraday calls the equatorial, to distinguish it from the position assumed by an iron bar under the same circumstances, and which he terms the axial.