Mutual inductance happens between two inductors, as shown in Figure 4.The magnetic field produced by coil (L₁) intersect with coil (L₂), and induced current flow in circuit 2. Therefore, the current in circuit 2 produce a magnetic field, and intersect with coil (L₁), and induced current in circuit 1. This exchange interaction is called mutual inductance (M), as shown in Figure 5

:

Figure 5

The magnetic flux in circuit 2 due to circuit 1 is

Φ B₂ = L₂i₂ + i₁ M₁₂ ………………………. (6-8)

and the magnetic flux in circuit 1 due to circuit 2 is

Φ B₁ = L₁i₁ + i₂ M₂₁ ……………………… (6-9)

Normally, the mutual inductance is the same for both circuits:

M₁₂ = M₂₁

The common example of mutual inductance is a transformer.