CHAPTER XXI.
MAGNETISM.
THE LOADSTONE—MAGNETIC CURVES—THE MAGNETIC NEEDLE—THE MARINER’S COMPASS—MAGNETO-ELECTRICITY.
We have already mentioned some of the properties of the loadstone or magnet; but as we are now about to enter more fully into the considerations of its attributes and of the compass, etc., we will add some further interesting particulars.
Ancient writers (Pliny, Homer, and Aristotle) mentioned the existence of the magnet, and Humboldt refers to the knowledge of it possessed by the ancients. Pliny says “the magnet-stone is found in Cantabria,” and we have heard of the loadstones that are supposed to support Mahomet’s coffin at Medina. The origin of this fable was (probably) owing to the order given by Ptolemy to his architect, Dinochares. Ptolemy wished the roof of a temple at Alexandria to be roofed with the magnet-stone, so that the own image of his sister, Arsinoe, should remain suspended therein. But the death of the king and his architect prevented the project from being carried out.
The name “magnet” is said to have been derived from a shepherd named Magnes, who, when tending his flock on Mount Ida, found that his iron crook was attracted to a certain stone; and six hundred years before the Christian era Thales wrote respecting amber and the magnet; and because they attracted various substances, he supposed they possessed life and power. They were the germs of the science now so developed in their applications, and whose full powers we are scarcely yet acquainted with.
We may remark that other bodies besides iron and steel are capable of magnetization; nickel and cobalt have the like property. Magnetism, properly so called, treats of certain bodies known as Magnets, describes the properties they possess, and the influence of magnetic force upon other substances. Electricity and magnetism are always associated, but practically the force is the same, electricity being the current or motive power, so to speak; and it is to Faraday that the world is indebted for the discovery of magneto-electricity.
Epinus’ theory of magnetism was that all bodies possessed a substance he termed magnetic fluid, the particles of which repelled each other. But while supposed to repel each other, they attracted particles in other bodies. Thus they attracted iron. Coulomb asserted that there were two fluids—a north and south fluid. Ampère’s theory was that magnetic bodies are made up of molecules, round which currents are always circulating in all directions when non-magnetized, but when magnetized the currents all flow in the same direction. The space through which a magnet diffuses its influence was called by Faraday the Magnetic Field. The lines of magnetic force will be understood from the accompanying illustration (fig. 263). If we cover a magnet with a paper and scatter iron filings over it, we shall see the manner in which the filings arrange themselves. They radiate in curves from the poles of the magnet, and are dependent upon its form. If it be a straight bar magnet, evenly magnetized, they will turn inward in oval curves.
