Mutual Inductance between Two Coils #8211; (i)If two coil (1 and 2) also called primary and secondary coils are placed close to each other (maximum coupling), then mutual inductance between them [M = B<sub>1</sub> N<sub>1</sub> N<sub>2</sub> A<sub>2</sub>/I<sub>1</sub>] (ii) If the coils are two concentric coplaner circular coils, then mutual inductance between them [M = πμ<sub>0</sub> N<sub>1</sub> N<sub>2</sub> r<sup>2</sup>/2R] ⇒ M ∝ r<sup>2</sup>/R (iii) For two solenoids tightly wound to each other as shown below Magnetic field inside the primary solenoid, B<sub>1</sub> = μ<sub>0</sub> n<sub>1</sub> I<sub>1</sub> (n<sub>1</sub> = N<sub>1</sub>/I) Thus, [M = μ<sub>0</sub> N<sub>1</sub> N<sub>2</sub> A/l]