What type of core is used to make an electromagnet?

$(a)$ 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.

Under what conditions is a permanent magnet obtained if a current-carrying solenoid is used? Support your answer with the help of a labelled circuit diagram.

In domestic electrical wiring,what is the red-colored insulated wire used for?

The material of the core of a strong electromagnet is:

How does the magnetic field produced by a current-carrying straight wire vary with the distance from the wire?