Iron and its derivative, steel, can be given the property of attracting other pieces of iron and steel. This property, known as magnetism, is possessed to a much lesser degree by nickel, cobalt, and gadolinium. Iron and steel combined with these and other magnetic materials will yield an alloy with much greater magnetic strength.

The magnetic effects of magnets are concentrated at areas called poles. These poles are of two types and have been designated as north and south poles because of the fact that a magnet supported freely in air will align its axis in a north-south direction.

The end of the magnet that points geographically north is called the north (N) pole, and the other end is called the south (S) pole. Although all materials have some degree of a magnetic property, most materials do not have a useful amount of this property and, for all practical purposes, can be called nonmagnetic.

Hard steel is used for the construction of permanent magnets. Soft steel is easier to magnetize, but will retain a relatively weak degree of magnetization when the magnetizing force is removed. This small amount of magnetism retained by soft steel is known as residual magnetism and is both desirable and important in the operation of electrical equipment

A very powerful temporary magnet can be made by placing a bar of soft steel inside a coil of wire carrying an electrical current. The intense magnetic force created is reduced to a weak residual force as soon as the current is interrupted. An electromagnet also can be used to magnetize magnetic materials by placing the material across the poles of the electromagnet.

Magnetic materials also can be magnetized by placing them near a magnet. The magnetism produced in the material by this method is called induced magnetism. In the case of soft steel, the effect is only temporary. The magnetism is lost as soon as the magnet is removed.

If two magnets are brought near each other, the following will result:
• like poles repel.
• unlike poles attract.

Magnets influence one another at a distance without actually making contact. The space around a magnet through which this invisible force acts is known as the magnetic field. The force itself may be represented by magnetic lines of force that are assumed to exist in the space between the poles of the magnet. These invisible lines, collectively referred to as magnetic flux. Magnetic lines of force cannot be blocked or insulated, but will pass through or within any material.

Field Strength
The concentration of lines of force is an indication of the magnetic strength at various points in the magnetic field. This concentration, often referred to as the flux density, is the number of flux lines in a square inch of CSA. In other words, as the number of flux lines per CSA increases, the magnetic field becomes stronger.

Properties of Magnetic Flux
The following accepted properties of magnetic flux are very useful in explaining the operation of a wide variety of electrical equipment using magnetic circuits:
1. There is no insulator for magnetic flux; it passes through all materials.
2. Lines of force are closed loops passing through the magnet and the space around it.
3. The loops, formed by the lines of force, tend to become larger and increase in length as they develop away from the magnet.
4. Lines of force have direction. They emerge from the N pole and enter the S pole.
5. Lines of force never cross one another.
6. Lines of force concentrate at the poles and develop maximum field strength there.
7. Large numbers of flux lines are easily established in magnetic materials, but are difficult to establish in nonmagnetic materials such as air.

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