1. The problem statement, all variables and given/known data This isn't exactly a homework question so there aren't any data/numbers to work with. I just want to know what to do if there are two ferromagnetic cores inside a coil of wire, arranged as concentric cylinders. Or how to account for the air gap between the core and the coil? 2. Relevant equations L = μN2A/l 3. The attempt at a solution Should it be treat as a series circuit? Or maybe parallel? Or do you weight the contributions of the relative permeabilities by the fraction of the area inside the coil they occupy?