1. The problem statement, all variables and given/known data We have a toroidal coil, and an infinite wire that runs through it on its symmetry axis. (See picture) We need to find the mutual inductance between the coil and the wire in two ways: 1. By running a current through the wire, and finding the induced voltage in the toroidal coil. 2. By running a current through the toroidal coil, and finding the induced voltage in the wire 2. Relevant equations 3. The attempt at a solution So, I manage when trying to solve (1): If there's a current in the wire, there's a magnetic field in the Theta direction, through the coil, and we can find L. However, if I try to solve it through (2): If I run a current through the coil, then the magnetic field would exist only inside the coil! So how can I find the flux on the wire, if it's outside the coil?... It would just be zero all the time, wouldnt it? Any help would be appreciated!