Acoaxial cable. a small solid conductor with radius a is supported by insulating, nonmagnetic disks on the axis of a thin-walled tube with inner radius b. the inner and outer conductors carry equal currents i in opposite directions. (a) use ampere’s law to find the magnetic field at any point in the volume between the conductors. (b) write the expression for the flux db through a narrow strip of length l parallel to the axis, of width dr, at a distance r from the axis of the cable and lying in a plane containing the axis.