ELECTRO MAGNETISM.

Almost immediately after Oerstedt's discovery, Arago and Davy independently discovered how to magnetize iron and steel by causing currents of electricity to circulate round them in spiral coils of wire. The method is shown in the simple diagram of Fig. 11 where a current from a single cell is passed through a spiral coil of wire, in the hollow of which

is placed a bar of iron or steel, which is thereby magnetized. The separate turns of the coil must not touch one another or the central bar, otherwise the current will take the shortest road open to it and will not traverse the whole of the coils. To prevent such short-circuiting by contact the wire of the coil should be overspun with silk or cotton, or covered with a layer of gutta-percha.

ELECTRO MAGNETISM.

All dynamic-electric machines are based upon that branch of electric science known as Electro Magnetism, hence this discovery marks one of the most important epochs in the progress of practical electricity.

This exceedingly important elementary fact in electro inagnetism can be shown in a variety of ways.

When a wire in which the electric current is flowing is brought near a compass or other magnetic needle, the latter will be affected and tend to change its position and set itself at right angles to the conductor. This proves that an electric current flowing in a wire is in itself a form of a N. of magnet.

Lines of Magnetic Force.-If a thin piece of paper is placed over a bar magnet and fine iron filings are sprinkled over it, the particles of iron will arrange themselves in regular curves between the poles and to map out or define lines in the magnetic fields which scientists call lines of force. See Fig. 12.

Fig. 10 exhibits the manner in which the filings arrange themselves about the ends of a horse shoe magnet.

The forms of the curves show not only the direction of the magnetic force, but they also enable us to draw conclusions as to its intensity. When the force is great the curved lines are thick and sharply defined, and when it is weak the lines are thin and less plain.

The lines of force are also to be found in the neighborhood of wires through which electric currents are passing. They are the outward effect produced by the passage of an electric current, but the most singular fact is that they can also be the cause of an electric current.

ELECTRO MAGNETISM.

Directions of Magnetic Force.-The direction of the magnetic force in a magnetic field may be defined as the direction in which a small pivoted magnetic needle points when held in the field at that point. If a small suspended magnetic needle or pocket-compass be placed at various points in the magnetic field surrounding a bar magnet, as represented in

Fig. 13, it will be found that the needle always points in a definite direction, which direction varies with its position in the field, the direction of the magnetic axis of the needle at any point representing the direction of the magnetic force at that point.

If a magnetic needle, similar to that in the above experiment, be suspended by means of a thread over a bar magnet, and moved from the north to the south pole of the magnet, as illustrated in Fig. 14, the centre of the needle will trace out curving lines connecting the two poles. The paths or lines followed by the centre of the magnetic needle are

ELECTRO MAGNETISM.

termed lines of magnetic force, and in the modern conception of a magnetic field this latter is assumed to be entirely filled up with these imaginary lines of force. These lines of force are assumed, for reasons to be hereafter understood, to have a certain positive direction, namely that direction in which a small north-seeking magnetic pole would tend to

move if placed in the magnetic field; or, in other words, the lines of force are assumed to stream or flow in a direction from the north to the south pole, as indicated by the arrows in Fig. 14.

Points Relating to Electro-Magnets. Electro-magnets are far more powerful in proportion to their size than steel magnets and can be made of any required size, those made for power stations sometimes weighing several tons.

Electro-magnets are used in nearly all electrical instruments not only because they are stronger than permanent magnets

ELECTRO MAGNETS.

but because they can be made to act instantly by passing a current of electricity through them at the most convenient moment.

The strength of an electro-magnet depends directly on the number of turns of wire and the current flowing through them.

In an electro-magnet with an iron core, the grade of the iron also affects the strength-the best soft Swedish iron furnishing the strongest magnetism.

To increase the amount of magnetism, due to a current in a wire at a certain point, it is only necessary to increase the length of wire about that point-by the operation of this law, the coil form of the magnet is evolved.

The Scientific American has given an account of a great electro-magnet made by Col. King several years ago at Willett's Point fortification, N. Y. The magnet core consisted of two old 15-inch guns, weighing 50,000 pounds each. It was turned into a club-footed magnet by the addition of many tons of heavy iron plates. The coil consisted of old torpedo cables 14 miles long, carrying 20 to 25 amperes. The armature consisted of 6 platform plates bolted together. A calculated force of 44,800 pounds was insufficient to tear off the armature, the chain used being broken by the strain. Five cannon balls,

NOTE.-Inasmuch as the field-magnets of dynamos are electro-magnets, the iron cores of which are excited by electric currents circulating in surrounding coils, it becomes a matter of primary importance to us to know what is the law that governs the electro-magnet. If we once know the relation that subsists between the exciting current and the magnetism that is produced by it, we can apply this knowledge to the design of dyna mos; for such knowledge will enable us to calculate beforehand the size of field-magnet and the number and gauge of coils that will be required in a dynamo that is to furnish any given amount of electric energy.