Magnetic field at a point d distance apart from a long straight current carrying wire is given by from Ampere’s circuital law. Where μ₀ is the magnetic permeability of free space, i denotes the current through the wire and d denotes the distance from the wire, of the point where magnetic field is to be calculated.

Now we know that, speed of light, (ϵ₀ denotes the electric permittivity of free space)

 c² = μ₀ ϵ₀

Thus we get magnetic field to be