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Consider two copper coils with N₁ and N₂ turns, labeled as primary and secondary coils, placed close to each other.

A variable current I₁ is passed in the primary coil, generating a magnetic field.

Some of the magnetic field lines also pass through the secondary coil, and the changing magnetic flux induces an emf and current in the secondary coil, which can be evaluated using Faraday's law.

Also, the magnetic flux in the secondary coil is proportional to the current flowing in the primary coil.

A constant M is introduced to remove the proportionality sign, which is called mutual inductance.

Mutual inductance only depends on the geometry and orientation of both the coils, and its SI unit is the henry.

On combining and rearranging the equations, the induced emf in the secondary coil is obtained.

Now, reversing the case, the induced emf in the primary coil due to the changing current in the secondary coil can be obtained.

If the coils are in vacuum, the corresponding constants of mutual inductance of both coils are equal.