Friends: I would like to verify my understanding of mutual induction. Coil 1: n1 turns per unit length, radius is r1, has a current I1 Coil 2: n2 turns per unit length, radius is r2, has a current I2 (in the same direction as I1) r1 > r2. Coil 2 is inserted inside Coil 1. What is the mutual inductance (per unit length)? I believe it is: M per unit length = μo*n1*n2*pi*(r2)^2 I think that the mutual inductance is the same for any of the following three scenarios: 1) Current I1 through Coil 1 only 2) Current I2 through Coil 2 only 3) Current I1 through Coil 1 and Current I2 through Coil 2 at the same time. Is this true?