The force of attraction or repulsion between the two poles of strengths m₁ and m₂ webers placed d metres apart in any medium of relative permeability μ_r is given by

Fmedium =  m₁m₂  
                4πμ_oμ_rd²

If the same pole i.e. of strength m1 and m2 are placed in air medium at the same distance i.e. d metres apart then force acting between hem is given by,

 Fair = m₁m₂
         4πμ_od²
 
Dividing expression (iv) by expression (iii) we get
                    m₁m₂
  Fair      =   4πμ_od²
Fmedium     m₁m₂
                 4πμ_oμ_rd²

Hence relative permeability of the medium (μ_r ) is defined as the ratio of the force between two magnetic placed at a certain distance in air to the force between them placed at the same distance in the medium.

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