MAGNETISM AND MAGNETS.

magnets are placed near to each other the N. pole of one is found to repel the N. pole and to attract the south pole of the other; and the reverse. It is precisely by this attraction and repulsion that motive power is produced by the agency of electricity.

The magnetic field is the space around the magnet in which the compass needle or other detector of magnetism will be affected. The magnetic field is said to be comprised of lines of force. These are infinite in number, and gradually become weaker and weaker, until they disappear as the distance from the magnet is increased.

The magnetic circuit is a closed circuit. The lines of force starting from the N. pole and entering again at the S. pole.

Magnetic Flux.-This term indicates the number of lines that pass through the magnetic current. It has the same meaning as the term "magnetic flow."

Magnetic Lag.-This is the tendency of hard iron and steel to take up magnetism slowly and part with it slowly. "Magnetic retardation" has the same meaning, also " magnetic inertia."

Magnetic Saturation is the greatest magnetic force which can be permanently imparted to a steel bar.

Residual Magnetism.--When a mass of iron has once been magnetized, it becomes a difficult matter to entirely remove all traces when the magnetizing agent has been removed, and, as a general rule, a small amount of magnetism is permanently retained by the iron. The magnetism so retained by the iron is known as residual magnetism, and it varies in amount

MAGNETISM AND MAGNETS.

with the quality of the iron. Well-annealed, pure, wroughtiron, as a rule, possesses very little residual magnetism, while, on the other hand, wrought-iron, which contains a large percentage of impurities, or which has been subjected to some hardening process, such as hammering, rolling, stamping, etc., and cast-iron possesses a very large amount of residual magnetism. This property of residual magnetism in iron is of great importance in the working of the self-exciting dynamo, and is, indeed, the essential principle of this class of machine.

The Magnetic Field.-It is now understood that the phenomena of magnetism are due to an atmosphere of magnetic influence which surrounds the poles, and to a lesser, the whole of the magnet. This atmosphere is termed the magnetic field.

Magnetic Tick.-When a bar of iron is suddenly magnetized or demagnetized, it emits a slight sound, called the "page sound" or the magnetic tick.

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NOTE.-If the North pole of a magnet is presented to the South pole of another magnet, they will attract and hold fast to each other; but, if a South pole is presented to another South pole or a North pole to a North pole, they will repel each other, and there will be no attraction.

MAGNETISM AND MAGNETS.

Strength of a Magnet.-The "strength" of a magnet is not the same thing as its "lifting power." The strength of a magnet is the strength of its poles. The strength of a magnet pole must be measured by the magnetic force which it exerts.

The lifting power of a magnet depends both upon the form of the magnet and on its magnetic strength. A horse-shoe magnet will lift a load three or four times as great as a bar magnet of the same weight will lift. The lifting power is greater if the area of contact between the poles and the armature is iucreased. Also the lifting power of a magnet grows in a very curious and unexplained way by gradually increasing the load on its armature, day by day until it bears a load which at the outset it could not have done. Nevertheless if the load is so increased that the armature is torn off, the power of the magnet falls at once to its original value. The attraction between a powerful electro-magnet and its armature may amount to 200 lbs. per square inch. (See fig. 15, page 46.)

Small magnets lift a greater load in proportion to their own weight than large ones. A good steel horse-shoe magnet weighing itself one pound ought to lift twenty pounds' weight. Sir Isaac Newton is said to have possessed a little lode-stone mounted in a signet ring which would lift a piece of iron 200 times its own weight.

USEFUL DEFINITIONS RELATING TO MAGNETS.

Magnets made of steel are usually called permanent magnets; these are of two forms, viz., the "bar magnet" and the "horseshoe magnet," which is merely the bar magnet turned around.

Artificial Magnet.-Any magnet not found in nature is an artificial magnet.

Horseshoe Magnet.-This is a magnetized bar of iron or steel bent in the form of a horseshoe or letter U. (See Fig. 10.)

Natural Magnet is a name sometimes given to a lodestone. Natural magnets are usually of irregular form, although they are sometimes reduced to regular shapes by cutting and grinding.

Compound Magnets consist of a number of single magnets separately magnetized, and afterwards bound together in bun. dles. Compound magnets are stronger in proportion to their weight than single magnets.

Permanent Magnet.-A magnet of hardened steel which retains its magnetism a long time after being magnetized. A permanent magnet will always attract and hold pieces of iron and steel. Its ends or poles are named North and South. There is usually a loose piece of steel or iron, called an "armature" put across the ends, which has the peculiar property of keeping the magnetism from becoming weaker.

A Polarized Electro-Magnet is one whose core is a permanent magnet. Such magnets are used in Duplex Telegraphy. The armature of this magnet is released only by a current in a fixed direction.

ELECTRO MAGNETISM.

FIG. 11.

Electro-magnetism is the foundation stone of commercial

electricity.

A magnet produced by passing an electric current through a wire conductor coiled around a bar of soft iron is called an electro-magnet; if the bar be of iron it will be a magnet only so long as the current flows, and electro magnetism is that science which relates to magnetism produced by the electric current, and which treats of the relation between electric currents and magnetism.

In 1820 Oerstedt made the important discovery that a conductor through which a current of electricity is passing acquires thereby all the properties of a magnet.

NOTE.-The great usefulness of the electro-magnet in its application to electric bells and telegraphic instruments lies in the fact that its magnetism is under the control of the current; when circuit is "made" it becomes a magnet, when circuit is "broken" it ceases to act as a magnet.