A magnet keeper, also known historically as an armature, is a ferromagnetic bar made from soft iron or steel, which is placed across the poles of a permanent magnet to help preserve the strength of the magnet by completing the magnetic circuit; it is important for magnets that have a low magnetic coercivity, such as Alnico magnets.
Keepers also have a useful safety function, as they stop external metal being attracted to the magnet. Most magnets do not need a keeper, only those with low coercivity, meaning that they are easily susceptible to stray fields. Magnet can be considered as the sum of many little magnetic domains, which may only be a few micrometers or smaller in size. Each domain carries its own small magnetic field, which can point in any direction. When all of domain are pointing in the same direction, the fields add, yielding a strong magnet. When these all point in random directions, they cancel each other, and the net magnetic field is zero.
In magnets with low coercivity, the direction in which the magnetic domains are pointing is easily swayed by external fields, such as the Earth’s magnetic field or perhaps by the stray fields caused by flowing currents in a nearby electrical circuit. Given enough time, such magnets may find their domains randomly oriented, and hence their net magnetization greatly weakened. A keeper for low-coercivity magnets is just a strong permanent magnet that keeps all the domains pointing the same way and realigns those that may have gone astray.
Magnetic lines of force and their properties
To describe the phenomena related to magnets, lines are used to depict the force existing in the area surrounding the magnet. These lines are called the magnetic lines of force. These lines do not exist actually, but are imaginary lines that are used to illustrate and describe the pattern of the magnetic field. As shown in the figure below, the magnetic lines of force are assumed to originate from the north pole of a magnet, then pass through the surrounding space and then arrive at the South Pole. Then these lines travel inside the magnet from the South Pole to the North Pole and hence complete the loop.
Lines of force are the lines in any such field the tangent of which at any point gives the field direction at that point and its density gives the magnitude of the field. Hence, magnetic lines of force are basically the lines of force which represent the direction of the magnetic field. The imaginary path traced by an isolated (imaginary) unit north pole may also be defined as a line of force. Magnetic lines of force are closed curves. Outside the magnet their direction is from north pole to south pole and inside the magnet these are from south to north pole.
They don’t have any origin or end and do not interact because if they do so then it would mean two value of magnetic field at a single point, which is not possible. At the poles of the magnet the magnetic field is stronger because the lines of force there are crowded together and away from the poles the magnetic field is week. i.e. magnetic field intensity depends on the number of lines of force. The number of magnetic lines of force passing through unit normal area is defined as magnetic induction whereas the number of lines of force passing through any area is known as magnetic flux. The lines of force can emerge out of the north pole of magnet at any angle and these can merge into the South Pole at any angle.
The direction of magnetic line of force is the direction of force on a North Pole, so the magnetic lines of force always begin on the North Pole of a magnet and end on the South Pole of the magnet. When a small magnetic compass is placed along a lie of force, it sets itself along the line tangential to it. Hence, the line drawn from the South Pole of the compass to its North pole shows the direction of the magnetic field.
Plotting the lines of force Terrestriel magnetism
The space around a magnet, where magnetic force can be felt by a magnetic body is called the magnetic field of that magnet. The field can be represented by lines, called magnetic lines of force. A magnetic line of force is a line, straight or curved, the tangent to which at any point gives the direction of the magnetic field at that point.
When two magnetic field i.e. magnetic field of earth’s magnet and bar magnet acts in the same place, the resultant field has a special character. When we placed the magnetic needle at any particular point, then the needle does not show any direction at that point as there is no net magnetic field at that point. This point is known as a null point or neutral point.
The given figure A and B show the neutral point. Here, in figure A, when north pole of the magnet points south and magnet in the magnetic meridian, the horizontal component of the earth’s magnetic field and bar magnet becomes equal and opposite in point X, which is the neutral point. In figure B, when the north pole of the magnet points geographic north, the neutral point lie towards the lateral side at point X.
A neutral point in a magnetic field is a point at which the horizontal component of earth’s magnetic field and the magnetic field due to the magnet are exactly equal and opposite. At the neutral point, the lines of force will not pass and compass needle will not point in any fixed direction. In other words, ‘A neutral point in a magnetic field is a point at which the horizontal component of earth’s magnetic field and the magnetic field due to the magnet are exactly equal and opposite. At the neutral point, the lines of force will not pass and compass needle will not point in any fixed direction’.