FIG. 345.-Apps's half-horse power Electro-Magnetic Engine.

THERMO-ELECTRICITY.

In 1820, Ersted, a Danish scientific man, discovered the connection between electricity and magnetism. It was not found where philosophers sought for it. They thought to imitate Nature; and as some steel knives were found to be powerfully magnetic after a discharge of lightning had passed through a box containing them, they subjected other pieces of steel to the discharge of powerful Leyden batteries without producing the effect they expected.

Ersted found that the electricity must be in motion, or in a dynamical state, such as it would be in when evolved from the voltaic battery.

Static electricity will, under certain arrangements to be hereafter described, magnetize steel; but the mere fact of allowing a wire charged with statical electricity (the force from the electrical machine) to approach a magnetic needle does not affect the needle like the same wire conveying a current from a single voltaic circuit or, still better, a battery.

M. Ampère, who took up the subject directly after Ersted had published his discoveries, laid the foundation of the science of electro-dynamics. He discovered that every part of the whole circuit-the wires, the terminals or poles, the battery, in fact, all parts-exercised a magnetic power upon the mag netic needle. He also proved that the force was in an eminent degree one of circulation. Ampère made himself fully understood by asking his readers to conceive a man lying down in the circuit, so that the wire lies along his face

and body. We are now to suppose that the current enters the wire at his feet and goes out at his head, and that his upturned face and eyes are directed to a magnetic needle suspended parallel with and over the wire conveying the electric current, so that the north pole of the needle points to his face. Directly the current passes, the needle is deflected to his left hand; and by reversing the direction of the current, and causing it to flow into the wire at his head and out from his feet, the needle will now move to his right hand.

FIG. 346.-Wire conveying a Current of Electricity affecting the Magnetic Needle.

Thus every possible variation may be imagined as long as we maintain the same relative positions of the wire and the human body; and it was further ascertained that the intensity of the electro-magnetic force is in the inverse ratio to the simple distance of the magnetic needle from the current; or, in other words, that the elementary action of a simple section of the current upon the needle is in the inverse ratio to the square of the distance.

If a single wire can affect a magnetic needle, it is evident that by doubling and trebling the wire, or winding it round in a helix, the effect must be enormously increased, provided the coils of wire do not touch each other, or are covered with some non-conducting material, such as silk or cotton; hence it is that coils of wire are constructed so that a piece of soft iron placed

FIG. 347.

inside the core becomes a most powerful magnet directly contact is made with the battery. When the immense power of the electro-magnet was ascer

tained, great anticipations were formed of the application of this force as a motive power. It is not surprising that this should have been the first conclusion. Thus the great electro-magnet, made by Mr. Apps, that heads the chapter on Dia-magnetism, will lift five hundredweight with a single quarterpint Grove's cell, and three tons with twenty cells. This conveniently arranged magnet, after being used for dia-magnetic experiments, may be employed for showing the attractive force of the great electro-magnet. It is attached to a lever, which turns it over; and, when suspended with the poles downwards, it is connected with a compound-lever arrangement, on the same principle as railway weighing-machines, and the weights used are one quarter, one half, and one hundredweight.

The writer well remembers the late Prince Consort, on the occasion of one of his private visits to the Polytechnic, putting a question to him as to the rate at which the electro-magnetic power increased or decreased with the distance from the great electro-magnet belonging to the Polytechnic. The attractive force diminishes enormously. Thus, in a paper read by Mr. Robert Hunt before the Institution of Civil Engineers, the following instructive diagram was exhibited:

It is shown that, whilst contact gave a power of 220 lb., at a distance of of an inch the attractive force diminished to 36 lb.

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FIG. 349.-A Dextrorsal and a Sinistrorsal Helix.

When a wire, traversed by an electric current, is held in iron filings, they

adhere to it as long as the current passes. If the wire is coiled upwards round a glass tube from left to right, it is called a dextrorsal helix; and if coiled downwards, and in the same direction, it is termed a sinistrorsal helix.

A piece of steel placed inside such a helix, conveying the voltaic current, is soon magnetized. If the same coil is used to convey the charge from a Leyden battery of 6 ft. surface, a piece of steel is instantly magnetized. Electricians had missed this form of the experiment until Ersted's discovery.

If a bar magnet be held so that it is horizontal, and the north pole directed to the vertical portion of the rectangular wire, so supported that whilst conveying the electric current it moves freely round in a circle (Fig. 350), it will be found

that, if the upright portion of the wire is conveying the current from below upwards, it is repelled, but attracted if the south pole is substituted; and thus, by the dexterous substitution of one pole for another in presenting the bar magnet to the rectangular wire, it may be caused to rotate.

Polarity is shown by the sides of the wire, whereas in steel magnets it is discoverable at the ends.

The same attraction and repulsion occurs if another electrified wire is brought towards the suspended rectangular wire whilst conveying the electrical

current.

Fig. 351 is a good illustration of the direction of the current circulating around each section of a magnet everywhere in the same direction, viz., from top to bottom in the face that is turned towards the moving wire, and from bottom to top in that which is opposite to it. The sum of these directions

amounts to a current.

A similar result is obtained when a horizontal wire is directed to a magnet suspended vertically. The magnetic currents circulating around the magnet are again shown by arrows. A magnet may, therefore, says De la Rive, be considered as formed by an association of electric currents, all circulating in the same direction around its surface, and all situated in planes parallel to each other, and perpendicular to the axis of the magnet. It is this hypothesis of Ampère of the constitution of magnets, shown in Figs. 351 and 352, and which

FIG. 353.

FIG. 352.-Magnet suspended in a perpendicular line, the Current flowing horizontally.

explains Ersted's original experiment, and also all those that relate to the deviation. In order to confirm the hypothesis to which he had been led, of the nature of magnetism, Ampère endeavoured to arrange electric currents in the same manner as he had conceived they were naturally arranged in a magnet.

FIG. 354.--Magnet revolving around Wire conveying the Current.

Thus a flat spiral coil of wire (Fig. 353), nicely supported and resting on points, and perfectly mobile, takes a position perpendicular to the magnetic meridian. By reversing the experiment, and causing the wire to be fixed, and the magnet