In case of uniform current distribution inside of a long wire, magnatic field increases linearly inside of the conductor and decreases inversely with radius outside of the conductor. For outside of the conductor, we can assume a coaxial cable with infinite outer radius. In this case the square of the magnetic flux decreases with inver square of r and integration from Ri to Ro(infinite) gives finite value per unit length of wire.

In case of uniform current distribution inside of a long wire, magnatic field increases linearly inside of the conductor and decreases inversely with radius outside of the conductor. For outside of the conductor, we can assume a coaxial cable with infinite outer radius. In this case the square of the magnetic flux decreases with inverse square of r and the volume integration from Ri to Ro(infinite) gives finite value per unit length of wire.