"Consider a conducting wire that runs along the central axis of a hollow conducting cylinder. Such an arrangement, called a coaxial cable, has many applications in telecommunications. In such a cable a current I runs in one direction along the hollow conducting cylinder and is spread uniformly over the cylinder's cross-sectional area. An equal current runs in the opposite direction along the central wire. How does the magnitude of B of the magnetic field outside such a cable depend on the distance r from the central axis of the cable
