A long straight wire of circular cross-section (radius a ) is...

The formula to calculate the magnetic field inside the conducting wire is given by

${B}_{\text{in}} = \dfrac{{\mu}_{0}Jr}{2} . . . ( 1 )$

The formula to calculate the magnetic field outside the conducting wire is given by

${B}_{\text{out}} = \dfrac{{\mu}_{0}J{a}^{2}}{2r} . . . ( 2 )$

From equation (1), it can be concluded that the magnetic field is directly proportional to the radial distance inside the wire, whereas from equation (2), it can be concluded that the magnetic field is inversely proportional to the radial distance outside the wire.