The illustration shows a permanent magnet magnetised with uniform magnetisation *M* (or magnetic polarisation *J*), surrounded by vacuum. The magnetic field lines are shown separately for each field: *M*, magnetic field strength *H* and magnetic flux density *B*, which is the vector sum of *M* and *H*.

The demagnetising field *H _{d}* points in the opposite direction but it is non-uniform because some magnetic flux closes also through the outside of the body (where

*M*= 0 and

*J*= 0).

As a result, inside the body *B* is also non-uniform and in terms of magnitude *B* < *J*.

Outside the body, the field lines of *B* and *H* have the same shape, because in vacuum the two vector quantities differ only by a scalar constant of the vacuum permeability μ_{0} (both *M* and *J* are zero).

This illustration also shows that at the boundary between the two media with different permeability values, for *H* the tangential component *H _{t}* is preserved, and for

*B*the normal component

*B*is preserved.

_{t}This image is based on (Fig. 1.1, p. 7):

demagnetising_field_m_j_h_hd_b_magnetica.png

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**S. Zurek, Encyclopedia Magnetica, CC-BY-4.0**

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