Analogue Recorders - Hysteresis cycle

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All magnetic material subjected to a magnetic field reacts according to a pattern which is described in the so-called hysteresis cycle. Every material with magnetic characteristics has its own typical diagram which describes its behaviour. In the following figure a general hysteresis cycle is shown:

Analogue recorders - Hysteresis cycle

Hysteresis cycle

  • A: all particles are aligned in random positions

  • B: positive saturation of the ribbon

  • C: residual magnetism (remnance), the ribbon has moved away from the head

  • D: absence of magnetism on the ribbon

  • E: negative saturation on the ribbon

  • F: residual negative magnetism

  • Br: remnance

  • Hc: coercivity, the quantity of magnetic field needed to delete a saturated magnetic ribbon

On the X-axis we have the magnetic field applied to the magnetic material (in our case, the particles on the magnetic ribbon), on the Y-axis we have remnance. To get a clear idea of the magnetizing rate, let's pretend to apply a sinusoidal magnetic field to the magnetic ribbon at a particular frequency. The following figure shows the sinusoid on which we have highlighted the points (A, B, C etc.) corresponding to the same phases in the hysteresis diagram and which we will now analyse one by one.

Analogue recorders - Magnetic field applied to the ribbon

Magnetic field applied to the ribbon

Let's start from point A, which corresponds to absence of magnetizing. In both diagrams we have 0 amplitude. Let's increase the applied magnetic field and reach point B of the sinusoid. On the hysteresis cycle we see the non-linear reaction of the magnetic particles which then follow the applied force until they reach point B, where the ribbon gets saturated. Now let's decrease the applied force and bring it back down to 0 (point C). Much to our surprise we notice that the remnance level hasn't reached 0 together with force but that the ribbon has remained magnetized. So this is the magnetic ribbons' distinguishing characteristic: they are able to memorize a magnetization even when the force that generated it has extinguished. As we follow down the sinusoid's path, we notice that to bring remnance back to 0 we need to apply a negative magnetic field. Then we reach point D where the remnance is absent. If we further increase the amount of negative magnetic field til we reach point E, a negative saturation will occurr. If we now decrease the negative applied magnetic field we reach point F (at point F magnetic field is lacking), characterized by a negative remnance. If yet again we increase the applied magnetic field, we manage to cancel the ribbon's magnetization (point G) and successively bring it back to saturation. The more the hysteresis cycle looks like a rectangle (remnance is very high when there is no applied magnetic field) the higher the ribbon's quality.


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