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In telegraphs on land the intensity of the battery or magnets is increased by induction on the same principle. It is by intensity that the electric current is enabled to pass through the wires, and that is augmented by increasing the number of coils round the cylinder: however, it is only advantageous when the distance between the stations is great, for then the resistance in the additional coils bears a small proportion to the resistance offered by very long wires, but a very great proportion to that opposed in very short ones. The nice adjustment for each case has been determined by the experiments of eminent electricians, and all the arrangements have been brought to great perfection in this wonderful triumph of science, which is due to Volta, who called into existence the fiery stream, and to Faraday, who has given it the energy of the lightning.

When the length of the wire in the helices of an electro-magnet is very great, it offers increasing resistance to the passage of the electricity, so that the cessation of magnetism is not instantaneous when the contact with the Voltaic battery is broken. To remedy that defect an instrument has been invented which instantaneously deprives the apparatus of the remaining electricity. A great length of fine wire gives the severest shocks, while a shorter and thicker wire gives the longest sparks and ignites the greatest quantity of platinum wire.

Ruhmkorff's electro-inductive apparatus has either been improved, or new machines constructed, by Messrs. Grove, Gassiot, and Joule, of intense energy. Indeed, so great is the energy of electro-induction, that hopes were entertained of its superseding steam as a motive power. For the current of electricity from an electro-magnet can be made to flow in opposite directions, so as to produce alternate attractions and repulsions, and consequently a continued motion, which might be applied as a motive force to machinery. However, Mr. Joule has proved that the power developed by one pound of coal in combustion is to that produced by one pound of zinc consumed in Mr. Grove's powerful electromagnetic apparatus as nine to one, so that, even if zinc were as cheap as coal, and a Voltaic battery as easily kept in order as an engine-furnace, electricity will not supersede steam as a motive power.

A current of electricity traversing a conductor gives out a quantity of heat determined by fixed laws, the amount of which is invariable as long as the machine to which it is applied remains

at rest; but the instant the machine is set in motion a reaction takes place in the intensity of the current, causing a diminution in the quantity of heat, because the heat that disappears is converted into the mechanical force exerted by the engine.

Mr. Joule's experiments prove that, whenever a current of electricity is generated by a magneto-electric machine, the quantity of heat evolved by that current has a constant relation to the power required to work the machine; and on the other hand, whenever an engine is worked by a Voltaic battery, that the power developed is at the expense of the calorific force of the battery for a given consumption of zinc, the mechanical effect produced having a fixed relation to the heat lost in the Voltaic current. The obvious conclusion Mr. Joule draws from these experiments is, that heat and mechanical power are convertible into one another, and it becomes evident, therefore, that heat is either the vis viva or living force of ponderable particles, or a state of attraction and repulsion capable of generating vis viva (N. 222).

SECTION XXXIII.

Electricity produced by Rotation - Direction of the Currents - Electricity from the Rotation of a Magnet-M. Arago's Experiment explained Rotation of a Plate of Iron between the Poles of a Magnet - Relation of Substances to Magnets of three Kinds - Thermo-Electricity.

M. ARAGO discovered a source of magnetism in rotatory motion. If a circular plate of copper be made to revolve immediately above or below a magnetic needle or magnet, suspended in such a manner that it may rotate in a plane parallel to that of the copper plate, the magnet tends to follow the circumvolution of the plate; or, if the magnet revolves, the plate tends to follow its motion; so powerful is the effect, that magnets and plates of many pounds weight have been carried round. This is quite independent of the motion of the air, since it is the same when a pane of glass is interposed between the magnet and the copper. When the magnet and the plate are at rest, not the smallest effect, attractive, repulsive, or of any kind, can be perceived between them. In describing this phenomenon, M. Arago states that it takes place not only with metals, but with all substances, solids, liquids, and even gases, although the intensity depends upon the kind of substance in motion. Experiments made by Dr. Faraday explain this singular action. He found that, if a piece of metal or a metallic wire forming a circuit of any form be moved from right to left across the lines of force proceeding from the pole of a bar magnet, these lines of force induce a current of electricity flowing in one direction; and when the motion of the metal or wire is reversed, the direction of the current is reversed also: the rotation of the magnet about its axis has no effect on these results, and no current is induced when the metal or wire is at rest. A plate of copper, twelve inches in diameter and one fifth of an inch thick, was placed between the poles of a powerful horseshoe magnet, consequently crossing the magnetic lines of force at right angles, and connected at certain points with a galvanometer by copper wires. When the plate was at rest no effect was produced; but as soon as the plate was

made to revolve rapidly the galvanometer needle was deflected sometimes as much as 90o, and by a uniform rotation the deflection was constantly maintained at 45o. When the motion of the copper plate was reversed, the needle was deflected in the contrary direction, and thus a permanent current of electricity was evolved by an ordinary magnet. The intensity of the electricity collected by the wires, and conveyed by them to the galvanometer, varied with the position of the plate relatively to the poles of the magnet.

The motion of the electricity in the copper plate may be conceived by considering that, merely by moving a single wire, like the spoke of a wheel, before a magnetic pole, a current of electricity tends to flow through it from one end to the other. Hence, if a wheel be constructed of a great many such spokes, and revolved near the pole of a magnet in the manner of the copper disc, each radius or spoke will tend to have a current produced in it as it passes the pole. Now, as the circular plate is nothing more than an infinite number of radii or spokes in contact, the currents will flow in the direction of the radii if a channel be open for their return; and, in a continuous plate, that channel is afforded by the lateral portions on each side of the particular radius close to the magnetic pole. This hypothesis is confirmed by observation; for the currents of positive electricity set from the centre to the circumference, and the negative from the circumference to the centre, and vice versá, according to the position of the magnetic poles and the direction of rotation; so that a collecting wire at the centre of the copper plate conveys positive electricity to the galvanometer in one case, and negative in another; that collected by a conducting wire in contact with the circumference of the plate is always the opposite of the electricity conveyed from the centre. It is evident that, when the plate and magnet are both at rest, no effect takes place, since the electric currents which cause the deflection of the galvanometer are only induced by motion across the magnetic lines of force. When the plate is placed edgewise so as to be parallel to these lines of force, no revolution of it with the most powerful magnet produces the slightest signs of a current at the galvanometer. The same phenomena may be produced by electro-magnets. The effects are similar when the magnet rotates and the plate remains at rest. When the magnet revolves uniformly about its own

axis, electricity of the same kind is collected at its poles, and the opposite electricity at its equator.

The phenomena which take place in M. Arago's experiments may be explained on this principle. When both the copper plate and the magnet are revolving, the action of the induced electric current tends continually to diminish their relative motion, and to bring the moving bodies into a state of relative rest; so that, if one be made to revolve by an extraneous force, the other will tend to revolve about it in the same direction, and with the same velocity.

When a plate of iron, or of any substance capable of being made either a temporary or permanent magnet, revolves between the poles of a magnet, it is found that dissimilar poles on opposite sides of the plate neutralize each other's effects, so that no electricity is evolved; while similar poles on each side of the revolving plate increase the quantity of electricity, and a single pole end-on is sufficient. But when copper, and substances not sensible to ordinary magnetic impressions, revolve, similar poles on opposite sides of the plate neutralize each other; dissimilar poles on each side exalt the action; and a single pole at the edge of the revolving plate, or end-on, does nothing. This forms a test for distinguishing the ordinary magnetic force from that produced by rotation. If unlike poles, that is, a north and south pole, produce more effect than one pole, the force will be due to electric currents; if similar poles produce more effect than one, then the power is not electric. These investigations show that there are really very few bodies magnetic in the manner of iron. Dr. Faraday therefore arranges substances in three classes, with regard to their relation to magnets:—those affected by the magnet when at rest, like iron, steel, and nickel, which possess ordinary magnetic properties; those affected when in motion, in which electric currents are evolved by the inductive force of the magnet, such as copper; and, lastly, those which are perfectly indifferent to the magnet, whether at rest or in motion.

It has already been observed that three bodies are requisite to form a galvanic circuit, one of which must be fluid. But, in 1822, Professor Seebeck, of Berlin, discovered that electric currents may be produced by the partial application of heat to a circuit formed of two solid conductors. For example, when a semicircle of bismuth, joined to a semicircle of antimony, so as to form a

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