Read the text. Magnetism and electromagnetism

Magnetism and electromagnetism

Magnetism has been known to man for many centuries, and the Chinese are said to have been aware of some of its effects as early as 2600 B.C. Its first practical use, the magnetic compass, is credited to the Chinese and was introduced in Europe about A.D. 1200. Dr. William Gilbert (1540-1603) made further discoveries about magnetism and is also credited with being the first to publish records of his work. After Gilbert’s discoveries many scientists have made numerous contributions to the study of magnetism. The principles they discovered have made possible the many applications of magnetism as used in electrical and electronic equipment.

RELATION of MAGNETISM to ELECTRICITY

Magnetism is so closely related and so important to electricity that the two are often called twins. The study of electrical principles established the Ohm's law relations and presented methods for solving series and parallel circuits. The study of magnetism and the solutions of series and parallel magnetic circuits are quite similar to their electrical counterparts. Electricity is so dependent upon magnetism that without it very few of our modern devices would be possible. Without the aid of magnetism, it would be impossible to generate and transmit power in large enough quantities to meet the needs of our industrial and home use.

MAGNETISM. MAGNETS. MAGNETIC MATERIALS

Magnetism is generally defined as the property of power of a material to attract and hold pieces of iron or steel. While this is true, it would be better to consider magnetism as the study of all the properties and actions of magnets and magnetic materials.

A magnet is defined as a body that has the property of polarity and the power of attracting iron and steel. Magnetic materials are those which will be attracted to a magnet; they may or may not possess the property of polarity and may or may not have, the power of attracting other magnetic materials.

These two definitions lead to the conclusion that all magnets are magnetic materials but not all magnetic materials are magnets.

NATURAL MAGNETS

Centuries ago it was discovered that certain stones taken from the earth had two peculiar properties. One was that they possessed the power to attract and hold to them other bits of similar stones or iron. The other was that, when an elongated piece of this stone was suspended from a cord, it would always come to rest with one end pointing north. The Chinese were the first to discover and use this stone to aid in determining directions. However, its later common use in navigation resulted in the name of lodestone meaning leading stone. This substance taken from the earth is now called magnetite. The name magnet was given to the lodestone because large deposits of the stone were found near the city of Magnesia in Asia

Minor. These stones are called natural magnets because they possess magnetic power when taken from the earth.

Natural magnets no longer have any practical value, as it is now possible to manufacture powerful artificial magnets in a variety of shapes to meet definite requirements.

ARTIFICIAL MAGNETS

The lodestone possessed the property of being able to pick up bits of steel, and though each bit of steel could be attracted to the lodestone. it was found that there was no attraction between the various bits of steel before being touched by the lodestone. If a bar of steel is rubbed or stroked with a piece of lodestone, it will be found that (he steel bar has the same properties as the lodestone and is able to attract some bits of steel to it. A material magnetized in this manner is called an artificial magnet.

The magnet produced by rubbing with a lodestone would be weak in terms of modern magnets. Such magnets are now made by inserting the steel bar in a coil of wire that has an electric current flowing through it.

PERMANENT MAGNETS

If a piece of steel is hardened by heat treatment and is then made an artificial magnet by being placed in a coil of wire carrying an electric current, it will be found that the hardened steel will remain a magnet for a long time after the magnetizing force has been removed. This type of artificial magnet is called a permanent magnet.

TEMPORARY MAGNETS

If a piece of iron, soft steel, or nickel is made an artificial magnet by means of a coil carrying an electric current, it will be found that these materials will lose practically all their magnetism almost immediately after being taken away from the magnetizing force. This type of artificial magnet is called a temporary magnet.

USES OF MAGNETS

Temporary magnets are generally used where the magnet has a coil of wire wound around it and an electric current is flowing through the coil. Examples are generators, motors, transformers, electric bells, buzzers, telegraph sounders, relays, and microphones. Magnetic phonograph pickups, and deflection and focusing coils used with the picture tubes of television receivers.

Permanent magnets are used in compasses, earphones, loudspeakers, electric meters, electric tachometers, miniature-sized motors, etc.