What is meant by electromagnetic induction?

What is a solenoid?

Which instrument works on the principle of 'Electromagnetic induction'?

On which principle does an electric motor work?

$(a)$ 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.

How does the strength of the magnetic field at the centre of a circular coil of a wire depend on:
$(a)$ radius of the coil
$(b)$ number of turns in the coil.