Methods of preparing magnets
- Using electricity
Since these magnets can be made, they are usually referred to as temporary magnets. Electromagnets are magnets made using electricity and so are called temporary magnet since they only show their magnetic influence when there is electricity passing through it.
Magnets made by induction and stroking are also temporary magnets. This is because; they will lose their magnetic influence when the inducing or stroking magnets are removed. This means that magnets that always exhibit their magnetic influence and characters are known as permanent magnets.
Permanent magnets are usually made from steel while temporary magnets are made from iron. Iron nails become temporary magnets when they are attached to magnets. The nails lose their magnetism when the magnet is taken away. Materials such as steel are able to retain their magnetism for a long time.
The material to be magnetized such as an iron nail is attached to a bar magnet for some period of time. After some time, some pins are brought close to the iron nails.
It is seen that the pins become attracted to the iron nails. The iron nails is now behaving like a magnet as a result of its attachment to the bar magnet. Conclusion: Magnet can also be made by induction.
Making a Magnet by Stroking
A nail is placed on a flat surface and is stroke repeatedly in the same direction with the North Pole of the magnet. The pole is slide along the nail from end to the other and then lifted away from the nail in a large circle. It is returned to the starting end of the nail. A pin is brought close to the nail.
It is seen that the nail attracts the pin to itself.
This shows that the nail has now been magnetized because of the stroking by the bar magnet so it now shows magnetic influence on the pin. Magnet can also be made by stroking with a bar magnet.
A wire is wound around a card board tube to form what is called a solenoid. Leave the ends of the wire so that, they can be connected to a source of power such as a battery in an electrical circuit. The material to be magnetized is placed in the solenoid and the current is switched on for a short time. The needle is then taken out and some pins are brought close to it.
The pins become attracted to the needle for some time. This shows that the needle becomes magnetized so, the pins were attracted. Magnets can also be made using electricity.
Molecular theory of magnetism
A popular theory of magnetism considers the molecular alignment of the material. This is known as Weber’s theory. This theory assumes that all magnetic substances are composed of tiny molecular magnets. Any unmagnetized material has the magnetic forces of its molecular magnets neutralized by adjacent molecular magnets, thereby eliminating any magnetic effect. A magnetized material will have most of its molecular magnets lined up so that the north pole of each molecule points in one direction, and the south pole faces the opposite direction. A material with its molecules thus aligned will then have one effective north pole, and one effective south pole. An illustration of Weber’s Theory is shown in figure above, where a steel bar is magnetized by stroking. When a steel bar is stroked several times in the same direction by a magnet, the magnetic force from the north pole of the magnet causes the molecules to align themselves.
After conducting a magnetic particle inspection, it is usually necessary to demagnetize the component. Remanent magnetic fields can:
- Affect machining by causing cuttings to cling to a component.
- interfere with electronic equipment such as a compass.
- create a condition known as “arc blow” in the welding process. Arc blow may cause the weld arc to wonder or filler metal to be repelled from the weld.
- cause abrasive particles to cling to bearing or faying surfaces and increase wear.
Removal of a field may be accomplished in several ways. This random orientation of the magnetic domains can be achieved most effectively by heating the material above its curie temperature. The curie temperature for a low carbon steel is 770oC or 1390oF. When steel is heated above its curie temperature, it will become austenitic and loses its magnetic properties. When it is cooled back down, it will go through a reverse transformation and will contain no residual magnetic field. The material should also be placed with it long axis in an east-west orientation to avoid any influence of the Earth’s magnetic field.
It is often inconvenient to heat a material above its curie temperature to demagnetize it, so another method that returns the material to a nearly unmagnetized state is commonly used. Subjecting the component to a reversing and decreasing magnetic field will return the dipoles to a nearly random orientation throughout the material. This can be accomplished by pulling a component out and away from a coil with AC passing through it. The same can also be accomplished using an electromagnetic yoke with AC selected. Also, many stationary magnetic particle inspection units come with a demagnetization feature that slowly reduces the AC in a coil in which the component is placed.
A field meter is often used to verify that the residual flux has been removed from a component. Industry standards usually require that the magnetic flux be reduced to less than 3 gauss after completing a magnetic particle inspection.
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