Coaxial cable. A solid conductor with radius a is supported by insulating disks on the axis of a conducting tube with inner radius b and outer radius c (Fig). The central conductor and tube carry equal currents 1 in opposite directions. The currents are distributed uniformly over the cross sections of each conductor. Derive an expression for the magnitude of the magnetic field
(a) At points outside the central, solid conductor but inside the tube (a (b) At points outside the tube (r> c).