All magnetic materials, video tape coatings amongst them, may be regarded for practical purposes as consisting of an infinite number of tiny bar magnets, each with its own north and south poles. This is a simplification, but suits our purposes well. In the natural state, these bar magnets are randomly aligned within the material so that their fields cancel one another out, and no external magnetic force is present. To magnetise the material, be it a blank tape or a solenoid core, we have to apply an external magnetic force to align the internal magnets so that they sit parallel to one another, with all their N poles pointing in the same direction. When the external field is removed, most of the magnets remain in alignment and the material now exhibits magnetic properties of its own.

The relationship or transfer curve (Figure 21.2) between the external force and the retained magnetism on the tape is not linear. The transfer curve is a graph with the magnetising force ( H) plotted along the horizontal axis, and the stored flux density ( B) on the vertical axis. Our starting point is in the centre, point 0. Here, no external magnetising force is present, and the magnetic material s internal magnets are lying in random fashion hence zero stored flux. Let s suppose we now apply a linearly increasing external flux. The flux density (strength of magnetism) in the material would increase in a non-linear fashion as shown by...