What phenomenon describes Faraday inducing current between two coils on an iron ring?

The phenomenon Faraday observed between two coils on an iron ring is mutual inductance.

In Faraday’s 1831 ring experiment, two separate insulated coils were wound on an iron ring. When a battery circuit connected to one coil was switched on or off, a brief current appeared in the other coil, even though the circuits were not electrically connected. The changing current in the first coil changed the magnetic flux through the second.

The effect is more precisely called mutual induction, while mutual inductance is the property that quantifies how strongly one circuit induces an electromotive force in another. An iron core improves magnetic coupling by concentrating the flux shared by the coils.

This experiment became a foundation for the transformer. The key detail is that the current in the second coil appears during a change, not simply because the first coil carries a steady current. Self-inductance, by contrast, concerns an induced effect within the same coil whose current is changing.