Magnetic pole - an apparent surface or region in space which appears to emanate magnetic field.¹⁾²⁾

Magnetic poles are always created if there is a loop with electric current. By convention, the magnetic polarity is such that when looking at the loop if the conventional current flows anti-clockwise, then it is the north magnetic pole (N); when the current flows clockwise then it is the south magnetic pole (S).³⁾⁴⁾⁵⁾ This is related to the right-hand rule.

Opposite magnetic poles attract: N-S or S-N, like poles repel: N-N or S-S.

Magnetic poles N and S are created for any loop of electric current

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Opposite magnetic poles attract (north-south N-S), similar poles repel (north-north N-N, or south-south)

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Magnetic field lines in a solenoid (cross-section view)

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Magnetic compass with red colour on the north-seeking end of the needle ^{by A. Pingston, Public domain}

Earth's magnetic field has a magnetic south pole at its geographic North Pole

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By convention, the end of the magnetic compass needle which points to the Earth's geographic North Pole was referred to as “North-seeking”, or simply “north”. The capital letters are not used for magnetic poles in order to distinguish them from the geographic poles.

Therefore, at the geographic North Pole there is the magnetic south pole S, and geograpic South Pole is the magnetic north pole N.

The magnetic poles of Earth swapped over their location many times over the Earth history, and it is expected that another reversal is to happen relatively soon (on a geological scale).

Compasses around a coil with electric current

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The right-hand rule is a consequence of this convention. Also, because of the link between electric current and magnetic field of a magnetic dipole such as loop of current or solenoid, a compass needle reacts to the magnetic field and thus “magnetic poles” can be identified by their apparent location in space as well as their magnetic polarity.

However, it should be noted that for an empty coil the location of the poles is only apparent, because the magnetic field is continuous and thus there is no single point-like location which can be identified as the precise location of the poles.⁶⁾ With continuous movement of the compass there will be a continuous change of the position of the needle, as shown in the photograph of a real coil with current surrounded by several small compasses. The compasses placed inside the coil simply follow the same line of indications as those immediately outside.

The polarity is related to the conventional direction of the current flow, which is from plus to minus. However, in metallic conductors the main carriers of electric current are negatively-charged electrons which flow from minus to plus.

Internal magnetic poles in a magnetic material ⁷⁾

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Toroidal sample has an optimum magnetic shape

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The apparent magnetic poles can arise not only at the extreme ends of a given block of material or a solenoid, but also in any other region, depending on the distribution of magnetic field due to magnetisation of the material or the applied magnetic field. For example, as illustrated in the image, an apparent magnetic pole can be created between two anti-series coils, because the magnetisation at the two ends points in opposing directions.⁸⁾

The presence of magnetic poles is undesired for measurement of magnetic properties of soft magnetic materials.⁹⁾ If the poles are present then there is magnetic energy stored in the volume outside of the sample under test. Therefore, an optimum shape for performing magnetic tests is on a toroidal sample, which has a well-defined magnetic path, which can carry the magnetic field with minimum stray field, so with the magnetic poles virtually eliminated, such that the demagnetising field is also minimised.

However, for measurement of properties for materials such as electrical steels it is not possible to create samples which are of toroidal shape and other approaches are used, for example Epstein frame or single-sheet tester.

The presence of magnetic poles means that there will be non-uniformity of magnetic field, such that the magnetic field lines will be closed both through the surrounding volume (such as the surrounding air), as well as through the material itself. The polarity will be such that the magnetic field “generated” by the poles will oppose the internal magnetisation, hence the apparent magnetisation of the body will be lower than if the sample was closed (no magnetic poles, such as in a toroidal sample). For this reason this field is called demagnetising field H_d.

Demagnetising field $H_d$ in a ferromagnetic body magnetised with uniform magnetisation $M$ ¹⁰⁾

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Demagnetising field $H_d$ in a ferromagnetic body magnetised with uniform magnetisation $M$ ¹¹⁾

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In the past magnetic poles were perceived as sources (or sinks) of magnetic field, and they were useful concepts (to some extent) when approaching the magnetostatic calculations as an analogy to electrostatic.¹²⁾ By using the involved magnetic quantities it is possible to define magnetic pole strength, and the definitions differ depend on the employed system of units.

In SI system, magnetic pole strength p is expressed by the ratio of magnetic flux Φ emanating from that pole to the permeability of vacuum.¹³⁾

This equation can be related to magnetic dipole moment m:

In the CGS system, unit pole strength p is defined by the force and length. If two identical magnetic poles are placed 1 centimetre apart, and they act on each other with force of 1 dyne then then each pole has a strength of unity.¹⁴⁾

• Magnetism
• Right-hand rule