If two magnetic poles of strength m1 and m2 are separated by a distance r, then the force F acting between them is given by
                                     
The quantity μ used in the above expression is called the magnetic permeability of the medium between the poles. It is defined as the relative tendency of the magnetic lines of force to pass through the medium as compared to that in the vacuum. For vacuum, μ is taken as equal to 1. Similarly, for air, μ is taken as approximately equal to 1.

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