**Coulomb’s inverse square law** states that the force of attraction or repulsion between the two magnetic poles is directly proportional to the product of their pole strengths and inversely proportional to the square of the distance between them.

If m_{1} and m_{2} are the pole strengths of two magnetic poles separated by a distance of d in a medium, then

F ∞ m_{1}m_{2} and F ∞ 1/d^{2}

so, F ∞ m_{1}m_{2}/d^{2}

**F = k (m _{1}m_{2}/d^{2})**

where k is the constant of proportionality and K = μ/4π, where μ is the permeability of the medium.

but, μ = μ_{0} * μ_{r}

so, μ_{r} = μ / μ_{0}

where μ_{r }= relative permeability of the medium.

μ_{0} = permeability of free space or vacuum.

let, m_{1} = m_{2} = 1; and d = 1m

we know, K = μ/4π

In free space, μ_{0} = 4π * 10^{-7} Hm^{-1}

then, **F = (10 ^{-7} * m_{1 }* m_{2})/d^{2}**

so, **F = 10 ^{-7} N** (let, m

_{1}= m

_{2}= 1; and d = 1m)

Therefore, the unit pole is defined as that pole which when placed at a distance of 1 metre in free space or air from an equal and similar pole, repels it with a force of 10^{-1} N.