The Telephone / An Account of the Phenomena of Electricity, Magnetism, and Sound, as Involved in Its Action

by A. E. Dolbear

MAGNETS.

It is related by Nicander that among the shepherds who tended their flocks upon the sides of Mount Ida was one named Magnes, who noticed, that, while taking his herds to pasture, his shepherd's crook adhered to some of the rocks. From this man's name some have supposed the name magnet to have been derived. It is, however, generally believed to have received its name from the ancient city of Magnesia in Asia Minor, near which the loadstone or magnetic substance was found. This rock, which possesses the remarkable property of attracting and holding to itself small pieces of iron or steel, is now known to be one of the ores of iron, and is called magnetite by mineralogists. The iron is chemically combined with oxygen, and forms 72.5 per cent of its weight. There is another ore of iron, known as hematite, which contains seventy per cent of iron; but the difference of two and a half per cent of iron in the ore is enough to make the difference between a magnetically inert substance, and one which may be able to lift a mass of iron equal to many times its own weight.

Sir Isaac Newton is said to have worn in a finger-ring a small loadstone weighing three grains, which would lift seven hundred and fifty grains, which is equal to two hundred and fifty times its own weight. The most powerful magnet now known is owned by M. Obelliane of Paris. It can lift forty times its own weight. Large pieces, however, do not support proportionally greater weights, seldom more than one or two times their own weight.

There are in many places in the world immense beds of magnetic iron-ore. Such are to be found in the Adirondack region in Northern New York, and in Chester County, Pennsylvania. The celebrated iron-mines of Sweden consist of it, and in Lapland there are several large mountains of it. It must not be inferred, that, because the mineral is called magnetite, all specimens possess the property called magnetism. The large masses seldom manifest any such force, any more than ordinary pieces of iron or steel manifest it: yet any of it will be attracted by a magnet in the same way as iron will be. The most powerful native magnets are found in Siberia, and in the Hartz, a range of mountains in Northern Germany.

When a piece of this magnetically endowed ore is placed in a mass of iron-filings, it will be seen that the filings adhere to it in greatest quantity upon two opposite ends or sides, and these are named the poles of the magnet. If the piece be suspended by a string so as to turn freely, it will invariably come to rest with the same pole turned towards the north; and this pole is therefore called the north pole of the magnet, and the action is called the directive action. This directive action was known to the Chinese more than three thousand years ago. In traversing those vast steppes of Tartary they employed magnetic cars, in which was the figure of a man, whose movable, outstretched arm always pointed to the south. Dr. Gilbert affirms that the compass was brought from China to Italy in 1260, by a traveller named Paulus Venetus.

When a piece of hardened steel is rubbed upon a natural magnet, it acquires the same directive property; and, as the steel could be easily shaped into a convenient form for use, a steel needle has generally been used for the needle of a compass. The directive power of the magnet has been and still is of incalculable value to all civilized nations. Ocean navigation would be impossible without it, and territorial boundaries are fixed by means of it; but there are other properties and relations of a magnet, which have been discovered within the last fifty years, which are destined to be as important to mankind as that of the compass has been.

In 1825 William Sturgeon of Woolwich, Eng., discovered that if a copper wire were wound around a piece of soft iron, and a current of electricity sent through the wire, the soft iron would become a magnet, but would retain its magnetism no longer than while the current of electricity was passing through the coil. The magnetism developed in this way was called electro-magnetism, and the iron so wound was called an electro-magnet. The first electro-magnet was made by winding bare wire upon the soft iron. This method will not produce very strong magnets. In 1830 Prof. Henry insulated the wire by covering it with silk, and was the first to produce powerful magnets.

On a soft iron bar of fifty-nine pounds weight he used twenty-six coils of wire, thirteen on each leg, all joined to a common conductor by their opposite ends, and having an aggregate length of seven hundred and twenty-eight feet. This apparatus was found able to sustain a weight of twenty-five hundred pounds. This electro-magnet is now owned by Yale College.

The power of the electro-magnet is enormously greater than that of any permanent magnet. A permanent magnet made by Jamin of Paris, which is made up of many strips of thin steel bound together, and weighing four pounds, is able to support a weight of one hundred pounds; but Dr. Joule made an electro-magnet, by arranging the coils to advantage, that would support thirty-five hundred times its own weight, or one hundred and forty times the proportionate load of Sir Isaac Newton's ring magnet.