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The direction of the field lines depends on the direction of the current causing them!


If one now forms a conductor loop from the straight conductor considered so far and repeats the experiment with cardboard and iron powder with it, one recognizes that at both points, where the conductor loop is led through the cardboard, the known circular lines exist ( fig. 3.1).

With the knowledge of the directional assignment it becomes provable that in the area between the two points the field lines have a common direction.


his is also the case in a coil, which in principle consists of several interconnected conductor loops. This means, however, that inside such a coil the field lines of all individual windings have a common direction, which leads to the generation of a magnetic field in the longitudinal direction of the coil (fig. 3.2).

The two ends of the coil behave like the poles of a dust magnet. If there is now ferromagnetic material inside the coil, the field lines run for the most part in this material, whereby they are correspondingly crowded together. The line density (number of field lines per unit area) is therefore much lower in air than in ferromagnetic material. It depends on the "magnetic resistance" of a material, which is inversely proportional to its permeability. Permeability is thus a measure of the penetrability of a material for field lines.

For calculations, the relative permeability is usually used, which links the absolute permeability with that of the vacuum. However, this "permeability" of a material not only depends on the composition of the material, but also changes with different degrees of magnetization. It is very high for ferromagnetic materials, compared to air or non-magnetizable materials.

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