Early Experiments in Electricity.

"A shock was in this manner sent through a regiment of soldiers. At an early period in the progress of electrical discovery, M. Nollet transmitted a discharge through a series of 180 men; and at the convent of Carthusians a chain of men being formed extending to the length of 5,400 feet, by means of metalic wires extended between every two persons composing it, the whole series of persons was affected by the shock at the same instant.

Experiments on the transmission of the shock were made in London by Dr. Watson, in the presence of the Council of the Royal Society, when a circuit was formed by a wire carried from one side of the Thames to the other over Westminster Bridge.

One extremity of this wire communicated with the interior of a charged jar, the other was held by a person on the opposite bank of the river. This person held in his other hand an iron rod, which he dipped in the river. On the other side near the jar stood another person, holding in one hand a wire communicating with the exterier coating of the jar, and in the other hand an iron rod, This rod he dipped in the river when instantly the shock was received by both persons, the electric fluid having passed over the bridge, through the body of the person on the other side, through the water across the river, through the rod heid by the other person, and through his body to the exterior coating of the jar.

Familiar as such a fact may now appear, it is impossible to convey an adequate idea of the amazement bordering on incredulity with which it was at that time witnessed."

MAGNETISM AND MAGNETS.

FIG. 10.

MAGNETISM is that branch of science which treats of the nature and properties of magnets and magnetic fields.

The theory proposed to account for the magnetization of iron is as follows: Each little particle of iron is supposed to be, and to remain always magnetic. In an unmagnetized iron bar these molecular magnets

are arranged irregularly. These little molecules resist being turned out of their usual positions. Hence when a magnetizing force is brought to bear upon them, the first effect is to turn those little molecules round, whoses axes are already

NOTE.-The name magnet was given by the ancients to certain hard black stones found in various parts of the world, notably at Magnesia in Asia Minor, which possessed the property of attracting to them small pieces of iron or steel. This magic property, as they deemed it, made the magnet-stone famous; but it was not until the tenth or twelfth century that such stones were discovered to have the still more remarkable property of pointing north and south when hung up by a thread. This property was turned to advantage in navigation, and from that time the magnet received the name of lodestone (or "leading-stone"). The natural magnet or lodestone is an ore of iron, known to mineralogists as magnetite. This ore is found in quantities in Sweden, Spain, Arkansas, the Isle of Elba and other parts of the world; it frequently occurs in crystals, the usual form being the regular octahedron.

MAGNETISM AND MAGNETS.

most nearly in the direction of the magnetizing field. As the magnetizing force increases others are turned, increasing thereby the apparent magnetism; at last, all are turned with their poles in the direction of the magnetizing field.

When this is the case, no further application of magnetizing force, however great, can increase magnetization. This is the point of saturation. It is easily seen that shocks, or heating, or anything which loosens the molecules, tends to facilitate the acquirement of magnetization. On removing the magnetizing force, those molecules which have not been much strained out of their position, fall back to their old directions; but those which have been greatly strained have acquired a permanent magnetic set, and hence remain.

In the case of torsion or twisting of a wire, we have a similar behavior. A slight torsion within the limits of elasticity disappears when the twisting force is removed, but a violent torsion results in a permanent deformation, which does not disappear when the twisting force is removed.

This theory explains how iron can remain, as it were, super-charged or super-saturated with temporary magnetism. In the iron there is very little resistance to magnetization; that is to say, the molecules do not resist very strongly being turned in similar magnetic directions, and hence when the force is removed, there is very little restoring tendency to make them go back into irregular positions, but a little knock or twist imparts just the necessary disturbance, and causes the regularity to disappear, and with it the apparent magnetism.

MAGNETISM AND MAGNETS.

Magnetism is like electricity, it cannot be seen ; that it exists is assured by certain effects which it produces. Toy magnets shaped like a horseshoe exhibit the principles upon which magnetism acts; i. e., magnets attract and hold fast anything that is made of iron and steel, while they have no effect on brass, copper, zinc, gold or silver.

A magnetized body cannot be regarded as a source of energy in itself. Energy must be expended to magnetize the iron, and must also be expended to demagnetize it.

Magnetism produces electricity as well as electricity produces magnetism.

During the application of the magnetizing force, any blows or shaking tend to facilitate the acquirement of magnetism, but when removed from the field, the same causes tend to remove it. Heat operates in the same way. If a piece of steel is made red hot, then placed in a strong magnetic field and suddenly cooled, it acquires very strong permanent magnetism. Iron at a bright red heat is, however, not affected by a magnet. If a piece of iron, or better still, a piece of hard steel, be rubbed with a lodestone, it will be found to have also acquired the properties characteristic of the magnet; it will attract light bits of iron and steel.

This was all, or nearly all, that was known of the magnet until 1600, when Dr. Gilbert published a large number of magnetic discoveries in his famous work “De Magnete." He observed that the attractive power of a magnet appears to reside at two regions, and in a long-shaped magnet these regions, or poles, are usually at the ends. The portion of the

MAGNETISM AND MAGNETS.

magnet which lies between the two poles is apparently less magnetic, and does not attract iron filings so strongly; and all round the magnet, halfway between the poles, there is no attraction at all. This region Gilbert called the equator of the magnet, and the imaginary line joining the poles he termed the axis.

There is no insulator for magnetism. It penetrates everything known. The so-called protectors against magnetism are really only excellent conductors which form a short cut for the magnetism around the article to be protected.

If a sheet of glass, or wood, or paper, be interposed between a magnet and the piece of iron or steel it is attracting, it will still attract it as if nothing were interposed. A magnet sealed up in a glass tube still acts as a magnet. Lucretious found a magnet put into a brass vase attracted iron filings through the brass. Gilbert surrounded a magnet by a ring of flames, and found it still to be subject to magnetic attraction from without. Across water, vacuum, and all known substances, the magnetic forces will act; with the single exception, however, that magnetic force will not act across a screen of iron or other magnetic material. ïf a small magnet is suspended inside a hollow ball made of iron, no outside magnet will affect it. A hollow shell of iron will therefore act as a magnetic cage, and screen the space inside it from magnetic influences.

All magnetized substances whether permanently or tempo rarily magnetized have what is called polarity. The pole which tends to point northward when free to move is called the north pole. The other is the south pole. When two