CHAPTER VIII.
The magic swan—Properties of the magnet—The mariners’ compass—The process of magnetizing—The dip of the needle—Magnetic properties in various substances.
The magic swan—Properties of the magnet—The mariners’ compass—The process of magnetizing—The dip of the needle—Magnetic properties in various substances.
A magicianof former days had a figure of a swan, which floated on a vessel of water, round the rim of which were placed the twenty-four letters of the alphabet. Addressing the spectators, he was accustomed to ask for a name to be given him, and it was correctly spelt by the swan, as it moved from one letter to another till it had indicated the whole. A little philosophy, in this instance, produced repeatedly great astonishment. A magnetic bar was placed in the swan, and the performer had a powerful magnet concealed in his own dress, and the swan, of course, followed his motions. Thus, if he wanted the swan to spell “Selina,” he moved first to S, then to E, and so on, through the successive letters of that name, till the word was spelt. On one occasion, however, the performer was not a little disconcerted—the swan stopped in its course and refused to move. Again and again the effort was made, but it was utterly in vain; the magician could onlyacknowledge that some person was in the room aware of his secret, and counteracting his movements. Sir Francis Blake Delaval avowed himself to be the person: he produced a magnet which he had used on facing the performer as he stood at the table; the swan was, therefore, placed between two attractive instruments, and, of course, remained immovable.
A magnet may be described as a piece of iron, which possesses the property of turning towards the poles of the earth. This extraordinary quality does not necessarily belong to all specimens of iron in its native state, but only to one kind or variety called the oxide, on account of its union with oxygen in a particular condition. The possession of a special quality in this ore of iron was not discovered from its polarity, or power of turning to the poles of the earth, but from its property of attracting small pieces of iron, which are not magnetic; and hence it was called the loadstone.
There are many uses to which the magnet has been applied, and there is a probability of its being much more extensively employed; but its most important application is in the construction of the mariners’ compass, which renders it possible freely to traverse the ocean. There has been some controversy as to the discovery of the directive power of the magnet, and the invention of the compass. It was once supposed to have been unknown until about the thirteenth century, but it is now generally acknowledged that the Chinese were acquaintedwith the compass at least eleven hundred and fourteen years before the birth of Christ. At the commencement of the thirteenth century, it was certainly in use in Europe; for cardinal de Vitty mentions it with some particularity, in a work entitled “The History of the East,” where he says, “The iron needle, after contact with the loadstone, constantly turns to the north star, which, as the axis of the firmament, remains immovable, while the others revolve; and hence it is essentially necessary to those navigating on the ocean.” This shows that the compass was not invented in Europe, as commonly believed, by Gioia, a pilot, and a native of Pasitano, a small village, situated near Amalfi, who lived about the end of the thirteenth century, but, by him, it appears to have been made fully available for the purposes of navigation.
As used by sailors in the Mediterranean at that period, it was a very uncertain guide; for the compass then consisted of a magnetic needle attached to two straws on a piece of cork, floating on water in a basin, or glass vase. Gioia, therefore, placed the magnetic needle upon a pivot, so that it was free to move in any direction, and thus prevented that inconvenience and inaccuracy of observation which must have resulted from the motion of the needle floating on water, agitated by the tossing of the vessel. The magnetic needle was afterwards attached to a card divided into thirty-two points, called therose des vents, so that the direction in which avessel was sailing could be minutely determined, and the means of ascertaining it was no longer dependent on the accuracy of the eye in measuring distances. The mariners’ compass is still constructed in the same manner, but is inclosed in a box with a glass cover, and is thus preserved from the influence of the wind. Another improvement has been made in so suspending the box that, however the vessel may be pitched by the waves, and rolled from side to side, the needle remains in a horizontal position, and gives accurate indications of the direction in which the vessel is sailing.
In addition to the properties already mentioned, the loadstone has the power of communicating its virtues to any piece of hard iron or steel, and that, without diminution of strength; so that, if but one piece had been discovered, it would have been sufficient for the production of all the magnets that have ever been formed by man. Other means may be adopted of accomplishing this purpose. Take a bar of iron, and, striking it several times with a hammer, it will become magnetic. This experiment may be performed with a common poker. The magnetism thus communicated to a steel bar will be much increased in power, if it be supported on another bar during the process of hammering.
Gay Lussac, a French chemist of great celebrity, discovered a method of making magnets by a process so simple, that it may, in all cases, be applied successfully. Take a piece ofthin iron wire and suspend it in a vertical position. The earth itself being a magnet, induces a magnetic power in the wire. To render this permanent, twist the wire till it breaks, and a magnet will be obtained.
Mrs. Somerville, well known for her excellent philosophical works, made some experiments on the effect of solar light in the production of permanent magnetism. If half of a small sewing needle be covered with paper, and the exposed part be placed in the violet or indigo ray, magnetism will be induced, and the same effect will be produced in a smaller degree by the blue and green.
To describe but one more mode; magnets are readily made by what is called the single touch, and this is perhaps the most simple and most effective way of proceeding. Place the steel bar to be magnetized on a table, or any other convenient place, and, as nearly as possible, north and south, which position is called by philosophers, the magnetic meridian. This being done, draw over it perpendicularly a strong magnet. In this operation, it is necessary to begin at one end of the bar, and draw the magnet over its entire length, and then again in the same direction. It must not be drawn backward and forward, for the power communicated in one direction, would be destroyed by an opposite motion.
The following experiments are very instructive:—Suspend a magnetic needle by a silk cord, so that it will hang in a horizontal position.Then bring it over the centre of a large magnet lying upon a table, and it will still retain its position; but, as it is brought near to either end, it will be bent downwards, and, at the extremities, will be vertical. This experiment illustrates what is called the dip of the magnet. On the equator of the earth, the needle is horizontal, or nearly so, but as it is brought near the poles it dips, and over either magnetic pole would be vertical. The reason of this is evident from the former experiment: at the equator, each pole of the needle is attracted in an equal degree by the north and south poles of the earth; but, if we proceed northward, the north pole of the magnet will be more attracted than the south, and point towards it until at last it becomes vertical. The poles of the earth’s rotation, that is, the points which would form the terminations of its axis, did it revolve on one, are not the magnetic poles; nor is the equator of the earth the magnetic equator. They do not, however, greatly vary.
Take, also, a bar magnet, and, placing it upon a table, cover it with a sheet of writing-paper. Then sprinkle upon it some fine iron filings, and they will arrange themselves in very beautiful curves round the magnet, showing, as it is supposed, the circulation of the magnetic fluid. From this experiment, we learn that the magnetic power is greatest at the poles; and this is true in reference to the magnetism of the earth, which increases in power from themagnetic equator to the magnetic poles of the earth, as determined by a great variety of interesting and delicate experiments. Sir Graves C. Haughton has communicated a paper to the June number of Brewster’sPhilosophical Magazine, entitled “Experiments proving the common nature of Magnetism, Cohesion, Adhesion, and Viscosity.”
This paper contains two separate sets of experiments, the first of which relates to the attraction the magnetic needle has for various mineral, vegetable, and animal substances: and it is not a little remarkable that antimony and bismuth, as well as copper, tin, and cadmium, are, in these experiments, shown to have attractive powers for the magnetic needle; though, in those made by Dr. Faraday, he has ranged them amongst the class of dia-magnetics, that is, of those that exhibited repulsion. Arsenic, too, which he found so intractable, was made, in the present experiments, to assume the real magnetic character, that is to say, the power of attracting and repelling, by being kept for a short time in contact with a bar magnet. Iodine, likewise, was found, on bringing it near the needle, to be able to attract it.
In most of these experiments, the needle was made to attach itself to the substances by being forced towards them by a magnet, which was gently withdrawn after contact was so effected. In this way, and by a reference to the degrees of the compass traversed by the needle, a hair of the head, or a spark of diamond, can beaccurately measured. The strength of the needle in its movement on a pivot was ascertained by azimuths, of which a detailed account is given.
The remainder of the memoir, which is contained in a supplementary number of the Magazine, is devoted to a detail of about five hundred experiments, in which non-ferruginous needles were made, by a modification of the magnetic needle, of which they formed a portion, to attach themselves to the same substances as in the preceding experiments. Thus, for instance, needles of most of the remarkable metals, as well as of glass, were found to have a strong affinity for nearly every kind of substance, whether mineral, vegetable, or animal, if its density was greater than that of cork or charcoal. Brass surpassed all the metals in its power of attraction, and, what is most remarkable, the magnetic needle was the lowest of all in the scale, showing not much more than one-third of the attractive energy of soft iron. Every substance of a crystalline or vitreous character exhibited remarkable magnetic properties, and this could not be mistaken, as it might be heightened at pleasure by contact with either pole of a powerful magnet. Towards the close of the experiments, the curious discovery was made, that needles of ivory, mother-of-pearl, tortoise-shell, horn, etc., were singularly magnetic, and this is traced to the albumen and gelatine they contained; and the inference is drawn, from this and other facts, that thecohesive, adhesive, and viscous properties of bodies are owing to real magnetic qualities, and that, by drying, albuminous, gelatinous, and glutinous fluids constitute various kinds of glass, which view is supported by what takes place with the gelatinous hydrate of silicium.
“The preceding experiments,” says the writer, “include a vast variety of substances in the mineral, vegetable, and animal kingdoms, that exhibit such strong attractive affinities for one another, that, however much they may differ in their external appearances, and in their very natures, they are bound together by common bonds that connect them all into a single family; for we find the metal attaching itself to crystalline, animal, and vegetable substances; and, again, the crystal, whether we call it by the name of diamond, salt, or sugar-candy, connecting itself readily to metallic, animal, and vegetable bodies. In a similar way, animal bodies attach themselves to those that are mineral and vegetable; and, to complete the circle, the vegetable kingdom, by its woods, its gums, its lac, and its resins, is connected with them all.”