$(a)$ Define electromagnetic induction.
$(b)$ Two coils $P$ and $S$ are wound over the same iron core. Coil $P$ is connected to a battery and a key,and coil $S$ is connected to a galvanometer. Draw a suitable diagram of this arrangement and write your observations when:
$(i)$ current in the coil $P$ is started by closing the key.
$(ii)$ current continues to flow in coil $P$.
$(iii)$ current in coil $P$ is stopped by removing the key.
Explain the reason for such observations.

(N/A) Electromagnetic induction is the phenomenon of generating an electric current in a conductor by changing the magnetic field associated with it.
$(b)$ The diagram shows two coils $P$ (primary) and $S$ (secondary) wound on an iron core.
$(i)$ When the key is closed,the current in coil $P$ increases from zero to a maximum value. This causes a change in the magnetic field linked with coil $S$,resulting in a momentary deflection in the galvanometer.
$(ii)$ When the current flows steadily in coil $P$,the magnetic field associated with coil $S$ remains constant. Since there is no change in the magnetic flux,there is no induced current,and the galvanometer shows zero deflection.
$(iii)$ When the key is removed,the current in coil $P$ decreases to zero. This causes a rapid change in the magnetic field linked with coil $S$,resulting in a momentary deflection in the galvanometer in the opposite direction.