Write the principle of a transformer.

(N/A) transformer works on the principle of $Mutual \text{ } Induction$.
According to this principle, when the magnetic flux linked with a coil changes, an induced electromotive force $(EMF)$ is produced in the neighboring coil.
In a transformer, an alternating current $(AC)$ flowing through the primary coil creates a time-varying magnetic flux in the core, which is linked to the secondary coil, thereby inducing an alternating $EMF$ in the secondary coil.