Some of the phenomena of electricity are manifested upon so large a scale as to be thrust upon the attention of everybody. Thus lightning, which accompanies so many showers in warm weather in almost every latitude, has always excited in some individuals a superstitious awe, as being an exhibition of supernatural agency; and probably every one feels more or less dread of it during a thunder-shower, and this for the reason that it affects so many of the senses at the same time. The flash may be blinding to the eyes if near to us; the thunder may be deafening to the ears, and so powerful as to shake the foundations of the hills, and make the ground upon which we stand to sensibly move: these with the remembered destructive effects that have been witnessed, of buildings demolished and large trees torn to splinters in an instant, are quite sufficient to raise a feeling of dread in the strongest mind. In the polar regions, both north and south, where thunder-storms are less frequent, the atmospheric electricity assumes the form called the aurora borealis, or the aurora australis, according as it is seen north or south of the equator.

More than two thousand years ago it was noticed by the Greeks that a certain kind of a mineral which was thrown up on the shores of the Mediterranean Sea, when rubbed would attract light bodies, such as shreds of silk or linen and bits of paper. To this substance they gave the name of Elektron, and the property developed thus by friction was afterwards called electricity. In 1600 Dr. Gilbert, physician to Queen Elizabeth, published a book in which he described numerous experiments demonstrating that electricity could be developed by friction upon a great variety of substances, such as stones, gems, and resins. The first machine for developing electricity was made by Otto von Guericke of Magdeburg, about 1680. His machine consisted of a ball of sulphur about six inches in diameter, which could be rotated. If the dry hand were held against the sulphur while it was being turned in a dark room, the sphere appeared to emit light: it also gave out a peculiar hissing or crackling sound. Newton experimented a little with electricity, and noticed that the rubber was an important element in developing electricity. He does not seem to have given to the subject the same attention that he gave to some other departments of science. Had he done so, it is probable that he would have advanced the study a hundred years; that is to say, he would probably have left it at the place where it actually was in 1790. So great were his abilities that in one lifetime he made greater additions to human knowledge than all the rest of mankind had made during the preceding thousand years. In the month of June, 1752, Franklin made that memorable experiment which immortalized him. He flew his kite to the thunder-cloud, practically asking the question of the lightning whether or not it was identical with electricity. The lightning came down the wetted twine to his hand, and proclaimed its identity.

For the next forty years the natural philosophers in both Europe and America only rung the changes upon what was known. They flew kites to the clouds; they made and charged Leyden jars, and discharged them through wires and chains and circuits of clasped hands, and studied the attractions and repulsions manifested by electrified bodies; but they added nothing of importance in the way of experiments.

In 1791 Galvani, a professor of anatomy at Bologna, announced a manifestation of electricity that was new and of a remarkable character, having its origin in the muscles of animals, and so was called animal electricity. He had some frogs' legs prepared for eating; by chance they were placed near an electrical machine with which Galvani was experimenting, so that a spark would occasionally pass to the legs, when they would contract as often as a spark passed to them. The motion was first observed by his wife, who called his attention to the phenomenon; and he very soon discovered that the thighs of a frog, skinned and suspended, made a very good electroscope. While experimenting in this way he made another and more important discovery; namely, that, when the muscles and nerves of the frog's leg were touched by pieces of two different metals, the leg would contract as before. Alexander Volta, another Italian professor, who had invented the electrophorus, and was possessed of great experimental skill, now turned his attention to the experiment of Galvani, and very soon discovered that the origin of the electricity that moved the frogs' legs was not in the legs themselves, but in the metals used. The first form of the galvanic battery was the result of Volta's investigations, and was called the Voltaic pile. This pile consisted of alternate disks of zinc, flannel, and copper, piled one on top of the other in constant succession in that order. The flannel was moistened with salt and water, or with diluted sulphuric acid. When the first zinc was connected with the last copper by means of a wire, a powerful current of electricity was obtained. This form of battery is not in use at all now, as much more efficient means are known for producing electricity; but this in 1800, when it was first made known in England, was very startling, and was one of those surprises which have been so frequent since then in the history of electricity.

Surprising things were done by Sir Humphry Davy, with a large Voltaic battery. Water was decomposed, and the metals potassium and sodium were first separated from their compounds with oxygen. Bonaparte had offered a prize of sixty thousand francs "to the person who by his experiments and discoveries should advance the knowledge of electricity and galvanism as much as Franklin and Volta did," and of "three thousand francs for the best experiments which should be made in each year on the galvanic fluid." This latter prize was awarded to Davy.

After Davy's successes in 1806, there was nothing of importance in an experimental way added to the knowledge of electricity, until 1820, when Oersted of Copenhagen announced that "the conducting wire of a Voltaic circuit acts upon a magnetic needle," and that the needle tends to set itself at right angles to the wire. This was a kind of action altogether unexpected. This observation was of the utmost importance; and at once the philosophers in Europe and America set themselves to inquire into the new phenomenon. The laws of the motion of the magnetic needle when acted upon by a current of electricity traversing a wire were successfully investigated by M. AmpÈre of the French Academy. He observed that whenever a wire through which a current of electricity was passing was held over and parallel with a magnetic needle which was free to move, and therefore pointed to the north, if the current was moving towards the north, the north pole was deflected to the west; if the current was moving towards the south, the south pole of the magnet was deflected towards the west; and that in all cases the magnet tended to set itself at right angles to the current; also that this angular displacement depended upon the strength of the current. Thus originated the galvanometer, an instrument that not only detects the existence of an electric current, but enables us to determine its direction and its strength. Our present knowledge of electrical laws is due, in a very large measure, to observations made with this instrument. Of course it has been very much modified, and made almost incredibly sensitive: yet, in all galvanometers, the fundamental principle involved in their structure is that of the action of a current of electricity upon a magnet, which was first noticed by Oersted.

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.

THE GALVANIC BATTERY.

The original form of the galvanic battery as devised by Volta, and modified but little during thirty years, consisted of a cell to contain a fluid, which was usually dilute sulphuric acid, in which two plates of different metals were immersed: the metals used were generally plates of zinc and copper, or zinc and silver. Such plates, when first placed in the liquid, will give a very good current of electricity; but it will not last long. The reason of this is easy to understand. Whenever a current of electricity is generated by chemical action of a liquid upon two different metals, there is always some decomposition of the liquid, and this decomposition takes place upon the plates themselves; and the liberated gases adhere to the plates, and prevent further contact with the acid; at the same time, the gases themselves act upon the plates, and generate a current of electricity in the opposite direction. This will of course interfere with the first current; and very soon the battery is useless until the plates have been withdrawn from the liquid. This physico-chemical process that takes place in such a battery is called the polarization of the plates.

The accompanying figure will help one to understand the actions going on in a battery cell of the kind mentioned. Let Pt represent a plate of platinum, and Zn a plate of zinc, both placed in a vessel containing hydrochloric acid, which is also represented by the symbols HCl. As such molecules are extremely minute, there will of course be an immense number of them between the plates. The plates are now to be connected by a wire running between them through the air. As soon as these conditions are fulfilled, a hissing sound will be heard coming from the cell, and bubbles of gas will be seen to rise from the platinum plate: these bubbles prove upon analysis to be bubbles of hydrogen. At the same time the zinc will begin to dissolve, forming what proves by analysis to be the chloride of zinc; and at the same time a current of electricity travels through the wire from the platinum to the zinc. The quantity of electricity that is thus generated is strictly proportionate to the quantity of hydrogen liberated, which is also proportionate to the weight of zinc dissolved; and this, in turn, is proportionate to the surface of the metals exposed to the action of the acid. Now, it happens under such circumstances as the above, that the liberated hydrogen adheres very strongly to the platinum, as there is nothing for it to unite with chemically; and therefore the plate will very soon be visibly covered with bubbles, which may be scraped off with a feather or a swab, but only to have the same thing repeated.

This coating of bubbles will prevent the acid from touching the plate, and so practically diminishes the surface of it; but the quantity of electricity generated being proportionate to the surface exposed to the chemical action, it will be understood at once how such polarization of the plates must soon bring the battery to a standstill.

In 1836 Prof. J. F. Daniell of London contrived a battery, which has been called the Daniell Cell, in which the metal (copper) that had the hydrogen liberated upon it was separated by a porous cell from the zinc. The zinc was immersed in dilute sulphuric acid, and the copper in an acid solution of blue vitriol (copper sulphate). The porous cup did not prevent the electricity from passing, nor the decomposition from taking place; but the hydrogen, which in this case would have been liberated at the copper plate, at once united with oxygen there, which it got by decomposing the copper sulphate: hence water was formed, and copper was deposited upon the copper plate; and, being an excellent conductor, the battery would keep up a strong action for a long time.

Mr. Grove, also of London, in 1839 invented a battery which still goes by his name, in which the hydrogen plate is of platinum immersed in strong nitric acid, enclosed also in a porous earthen cell; and this, in turn, is plunged into a vessel containing dilute sulphuric acid and the zinc. In this case the liberated hydrogen immediately decomposes the nitric acid, which readily parts with its oxygen; water is the product, as in the other case, and the nitric acid loses strength. Strips of carbon have been substituted for the platinum, and this is called the Bunsen battery. It is otherwise like the Grove battery; it gives a very powerful and constant current and it is by the use of one or the other of these batteries, that most of the experiments in electricity are performed in institutions of learning, and, until lately, most in use for telegraphic purposes.