MAGNETISM.
The attractive power of the loadstone has been known from a very remote period. The natural magnet appears native in a grey iron ore in octahedral crystals, composed of 168 parts of iron, and 64 parts of oxygen, Fe3O4. Its properties seem to have been studied in Europe during the dark ages, and a directive power is alluded to by Cardinal James de Vitri, who flourished about the year 1200, who observed, that it was indispensable to those who travel much by sea.
In modern times, the history as well as the nature of the magnet has engaged remarkable attention; and it has been determined beyond all dispute that the magnet was used by the Chinese under the name of the tche-chy (directing-stone) about 2604 years before Christ. It passed from them to the Arabs, and was first used in Europe after the Crusades; and Ludi Vestomanus asserts, that about the year 1500 he saw a pilot in the East Indies direct his course by a magnetic needle like those now in use.
RELATION OF MAGNETISM TO ELECTRICITY.
The most remarkable theories have been invented to account for the phenomena of magnetism. Halley imagined magnetic globes to be moving to and fro in the interior of the earth. Barlow’s theory, which refers the whole to electrical currents, is the most rational. Dr. Faraday, for whose kindness to us in early days we always feel grateful, in a series of very curious experiments, has succeeded in identifying magnetism with galvanism, by directing galvanic currents at right angles to the direction of powerful magnets. And its connexion with this and the common species of electricity has deprived it of all its conjuring powers, and reduced it to the well-defined action of electrical bodies.
It is not for us to write either a history of this interesting subject, or to treat it as a science to be acquired; but as it embodies a great number of most instructive and amusing experiments, we think it proper to introduce it here.
TO MAKE ARTIFICIAL MAGNETS.
This may be done by stroking a piece of hard steel with a natural or artificial magnet. Take a common sewing-needle and pass the north pole of a magnet from the eye to the point, pressing it gently in so doing. After reaching the end of the needle the magnet must not be passed back again towards the eye, but must be lifted up and applied again to that end, the friction being always in the same direction. After repeating this for a few times the needle will become magnetised, and attract iron filings, &c.
HOW TO MAGNETISE A POKER.
Hold it in the left hand in a position slightly inclined from the perpendicular, the lower end pointing to the north, and then strike it smartly several times with a large iron hammer, and it will be found to possess the powers of a magnet, although but slightly.
TO SHOW MAGNETIC REPULSION AND ATTRACTION.
Suspend two short pieces of iron wire, N S, N S, so that they will hang in contact in a vertical position. If the north pole of a magnet N be now brought to a moderate distance between the wires, they will recede from each other, as in [figure 1].
The ends S S being made south poles by induction from the north pole N, will repel each other, and so will the north poles N N. This separation of the wires will increase as the magnet approaches them, but there will be a particular distance at which the attractive force of N overcomes the repulsive force of the poles S S, and causes the wires to converge, as in [figure 2]; the north poles N N still exhibiting their mutual repulsion.
NORTH AND SOUTH POLES OF THE MAGNET.
Each magnet has its poles, north and south—the north or south poles of one magnet, repel the north and south pole of another. If a magnet, as in the following [figure], be dipped in some iron filings, they will be immediately attracted to one end. Supposing this to be the north pole, each of the ends of the filings, not in contact with the magnet, will become north poles, while the ends in contact will by induction become south poles. Both will have a tendency to repel each other, and the filings will stand on the magnet as in the figure.
POLARITY OF THE MAGNET.
The best method of proving this is to take a magnet or a piece of steel rendered magnetic, and to place it on a piece of cork by laying it in a groove cut to receive it. If the cork be placed in the centre of a basin of water, and allowed to swim freely on its surface, so that it is not attracted by the sides of the basin, it will be found to turn its north pole to the north, and its south pole to the south, the same as the mariner’s compass. If you fix two magnets in two pieces of cork, and place them also in a basin of water, and they are in a parallel position with the same poles together, that is, north to north, and south to south, they will mutually repel each other; but if the contrary poles point to one another, as north to south, they will be attracted.
THE MAGNETIC FISH.
Fish are to be purchased at the toy-shops, by which the young “magnétique” may perform this experiment; they are made hollow, and will float on the water. In the mouth of each should be inserted a piece of magnetic wire. The angling rod is like any other rod, and has a silken thread for a line, and an iron hook also strongly magnetised. To catch the fish it is only necessary to put the hook in contact with the noses of the fish, and they will be taken without any of the baits mentioned in the former part of this work.
THE MAGNETIC SWAN.
The figure of a swan should be cut in cork, and within its beak a small strongly magnetised piece of steel should be placed. The swan should then be covered with a coating of white wax, and fashioned further into the shape of a swan, and glass beads may be placed in its head for eyes. This should be placed in a small tub or large basin of water, and to make it swim about, you should place in a white stick about nine inches long a magnetic bar, on which the north and south poles are marked. If you wish to bring the swan towards you, present to him the north pole of the wand, if you wish it to retire, present the south pole, and thus you may direct the swan to any part you desire.
TO SUSPEND A NEEDLE IN THE AIR BY MAGNETISM.
Place a magnet on a stand to raise it a little above the table; then bring a small sewing-needle containing a thread, within a little of the magnet, keeping hold of the thread to prevent the needle from attaching itself to the magnet. The needle in endeavouring to fly to the magnet, and being prevented by the thread, will remain curiously suspended in the air, reminding us of the fable of Mahomed’s coffin.
TO MAKE ARTIFICIAL MAGNETS WITHOUT THE AID EITHER OF NATURAL LOADSTONES OR ARTIFICIAL MAGNETS.
Take an iron poker and tongs, or two bars of iron, the larger and the older the better, and fixing the poker upright, hold to it with the left hand near the top P by a silk thread, a bar of soft steel about three inches long, one fourth of an inch broad, and one twentieth thick; mark one end, and let this end be downwards. Then grasping the tongs T with the right hand a little below the middle, and keeping them nearly in a vertical line, let the bar B be rubbed with the lower end L of the tongs, from the marked end of the bar to its upper end about ten times of each side of it. By this means the bar B will receive as much magnetism as will enable it to lift a small key at the marked end; and this end of the bar being suspended by its middle, or made to rest on a joint, will turn to the north, and is called its north pole, the unmarked end being the south pole. This is the method recommended by Mr. Caxton, in his process, which he regarded superior to those in former use, and of which a more detailed account will be found in his interesting volume.
HORSE-SHOE MAGNETS.
The form of a horse-shoe is generally given to magnetised bars, when both poles are wanted to act together, which frequently happens in various experiments, such as for lifting weights by the force of magnetic attraction, and for magnetising steel bars by the process of double touch, for which they are exceedingly convenient. The following is the method of making a powerful magnetic battery of the horse-shoe form. Twelve bars or plates of steel are to be taken, and having been previously bent to the required form, that is, the horse-shoe shape, they are then bound together by means of rivets at their ends; before being finally fastened they are each separately magnetised, and afterwards finally united.
Horse-shoe magnets should have a short bar of soft iron adapted to connect the two poles, and should never be laid by without such a piece of iron adhering to them. Bar magnets should be kept in pairs with their poles turned in contrary directions, and they should be kept from rust. Both kinds of magnets have their power not only preserved but increased, by keeping them surrounded with a mass of dry filings of soft iron, each particle of which will re-act by its induced magnetism upon the point of the magnet to which it adheres, and maintain in that point its primitive magnetic state.
EXPERIMENT TO SHOW THAT SOFT IRON POSSESSES MAGNETIC PROPERTIES WHILE IT REMAINS IN THE VICINITY OF A MAGNET.
Let M be a magnet and K a key, held horizontally near one of its poles, or near its lower edge. Then if another piece of iron, such as a small nail, be applied to the other end of the key, the nail will hang from the key, and will continue to do so while the magnet is slowly withdrawn; but when it has been removed beyond a certain distance, the nail will drop from the key, because the magnetism induced in the key becomes at that distance too weak to support the weight of the nail. That this is the real cause of its falling off may be proved by taking a still lighter fragment of iron, such as a piece of very slender wire, and applying it to the key. The magnetism of the key will still be sufficiently strong to support the wire, though it cannot the nail, and it will continue to support it even when the magnet is yet further removed; at length, however, it drops off.
ELECTRO-MAGNETISM.
The identity of magnetism with electricity alluded to in a [former paragraph], has led to the formation of a new science under the above name, and to some of the interesting experiments connected with it, we shall briefly allude for the amusement of the young reader.
POWER OF THE ELECTRO-MAGNET.
The same influence which affects the magnetic needle already described, will also communicate magnetism to soft iron. If a bar of that metal bent, as in the [drawing], be surrounded with a common bonnet wire, or a copper wire prevented from touching the iron by a winding of cotton or thread, and then if a current of voltaic electricity be sent through the wire, the bar becomes a powerful magnet, and will continue so as long as the connexion with the battery is preserved. On breaking the contact, the magnetism disappears. This experiment may be easily made by the young reader with a horse-shoe magnet, surrounded by several coils of wire. P is the positive, and N the negative pole.
THE MARINER’S COMPASS AND EXPERIMENTS WITH A POCKET COMPASS.
The mariner’s compass is an artificial magnet fitted in a proper box, and consists of three parts—1, the box; 2, the card or fly; and 3, the needle. The box is suspended in a square wooden case, by means of two concentric brass circles called gimbals, so fixed by brazen axes to the two boxes, that the inner one, or compass-box, retains a horizontal position in all motions of the ship. The card is a circular piece of paper which is fastened upon the needle, and moves with it. The outer edge of the card is divided into thirty-two points, as shown in the engraving, called points of the compass. The needle is a slender bar of hardened steel, having a hollow agate cup in the centre, which moves upon the point of a pivot made of brass.
VARIATION OF THE NEEDLE.
The magnetic needle does not point exactly north and south, but the north pole of the needle takes a direction to the west of the true north. It is constantly changing, and varies at different parts of the earth, and at different times of the day.
DIP OF THE NEEDLE.
Another remarkable and evident manifestation of the influence of the magnetism of the earth upon the needle is the inclination or dip of the latter, which is a deviation from its horizontal place in a downward direction in northern regions of its north, and in southern regions of its south pole. In balancing the needle on the card, on account of this dipping, a small weight or moveable piece of brass is placed on one end of the needle, by the shifting of which either nearer to or further from the centre, the needle will always be balanced.
USEFUL AMUSEMENT WITH THE POCKET COMPASS.
Pocket compasses are to be bought for five or six shillings, and may be used in many ways. In travelling over mountains or a wide extended moor, they are indispensably necessary; and no one should go a tour into Wales, Scotland, or the lakes without such a companion, and it will be a very useful and amusing exercise for any young person to take the bearings of his own or some particular locality, and make out what may be called a bearing card. This he may easily do in the following manner. Supposing he wishes, for instance, to take the bearings of his own house, he has nothing to do but to set his pocket compass upon a map of the district,—a county map will do very well, unless his house stands on the verge of a county, then two county maps will be necessary. He must make the north of the map exactly coincide with the north, as indicated by his compass, and having fixed his map in this situation, he should take a ruler and piece of paper, and dot down the exact bearings of each important town, or place, or village, around him. Let him suppose himself, for instance, in the town of Cambridge, and laying down his map as indicated by the compass, north to north and south to south, he will find the following places due north, Wilberton, Wentworth; Little Wilbraham, Teversham, due east; Duxford and Chesterfield, south; Coton and St. Neots in Huntingdonshire, west. The other points of the compass may be filled up in the same manner. Should therefore our young friend be upon any elevated situation near his own dwelling, or upon any other elevated spot from which the bearings have been taken, he will be able to inform his young friends that such and such a place lies in such a direction, that this place lies due north, the other north-west, a third south-east, the fourth south-west, &c. &c.
INTERESTING PARTICULARS CONCERNING THE MAGNET.
Fire-irons which have rested in an upright position in a room during the summer months are often highly magnetic.
Iron bars standing erect, such as the gratings of a prison cell, or the iron railings before houses, are often magnetic.
The great iron-clad ships, which have now replaced the wooden walls of Old England, are powerfully magnetic, and therefore affect the compass by which the vessel is steered; ingenious arrangements are therefore made to correct the effect of the local attraction, so that the man-of-war may be steered correctly.
Magnetism may be made to pass through a deal board; to exhibit which, lay a needle on the smooth part above, and run a magnet along the under side, and the needle will be found to follow the course of the magnet. A magnet dipped into boiling water loses part of its magnetism, which however returns upon its cooling.
A sudden blow given to a magnet often destroys its magnetic power.
