Significance: Mutual inductance allows electrical energy to be transformed from generator voltage to transmission line voltage, and back down for motors and lights.Measurement Challenges: Iron-core transformers have near-perfect coupling between primary and secondary windings. Because of this, ordinary measurements struggle to detect minute differences in mutual inductance. These differences are instead typically measured via secondary transformer characteristics (like voltage regulation, leakage reactance, or frequency characteristics).Coupling Types:Closely Coupled: Iron-core transformers.Loosely Coupled: Tuned circuits used in radio-frequency amplifiers, Neutrodyne receivers, and band-pass filters in heterodyne receivers.