1. The problem statement, all variables and given/known data The problem is simple: there is a coaxial cable where the current flows in one direction in the inside conductor and in the opposite direction in the outside conductor. I'm asked to find the magnetic field on a point situated in the space between the two (that space is filled with an electric insulator) and the magnetic field on a point situated in the middle of the outer conductor. 2. Relevant equations Ampere's Law 3. The attempt at a solution For the first, it's simple. I found the magnetic field created by each conductor using Biot-Savart's law and added them. For the second, I'm intrigued. Inside a conductor the electric field is zero, it acts as a Faraday's cage. What about the magnetic field?