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Many textbooks show a marked reluctance to mention magnetic poles. To some extent this is understandable since the pole model, like all scientific models, is limited in its application. In the case of the magnetic pole model, its limitations become clear when it is realised that the physical phenomena that the pole model is used to explain can also be explained by alternative models based, for example, on elecric currents (diamagnetism and paramagnetism) or intrinsic diople moments (ferromagnetism). Thus the magnetic pole model is quickly superceded by a sequence of more sophisticated models. Furthermore, magnetic poles do not exist in isolation - free magnetic poles have never been observed.  Magnetic pole models always involve equal and opposite pairs (usually as dipoles like in a but sometimes as a complicated systems of equal numbers of opposite poles).  The poles of a bar magnet are not located at a geometric points as assumed in the model. In the case of a long thin , however, the poles may br considered to be located at points at each end of the magnet.

On the other hand, many common phenomena can be explained quite easily by the pole model, whereas their alternative descriptions  can be very complicated. Examples of phenomena to which the pole model may be applied readily include the force that causes magnets to attract pieces or iron, steel, etc., the force between magnets and the behaviour of a compass needle. Magnetic pole models are widely used in and in .

Definition of magnetic pole strength

The definition of magnetic pole strength given in Section 16.3 of Understanding Physics is based on the experimental observation that the work done in moving a pole in a circular path once around a current is proportional to the current, that is

You will learn in Section 18.5 that the this result is observed to hold for any path around the current. Thus the definition of magnetic pole strength can be generalised as follows

where F is the force experienced by the pole as it is moved around a current I along any closed path C.