$A$ device used for producing current is:

Two coils $C_{1}$ and $C_{2}$ are wrapped around a non-conducting cylinder. Coil $C_{1}$ is connected to a battery and a key $K$,and coil $C_{2}$ is connected to a galvanometer $G$. On pressing the key $K$,current starts flowing in the coil $C_{1}$. State your observations in the galvanometer for the following cases:
$(i)$ When key $K$ is pressed on.
$(ii)$ When current in the coil $C_{1}$ is switched off.
$(iii)$ When the current is passed continuously through coil $C_{1}$.
$(iv)$ Name and state the phenomenon responsible for the above observation. Write the name of the rule that is used to determine the direction of the induced current.

The magnetic field produced in a straight conducting wire on passing the current through it is ............ .

State the observation made by Oersted on the basis of his experiment with current carrying conductors.

The material of the core of a strong electromagnet is:

By increasing the number of turns $3$ times in a toroidal solenoid,the magnetic field: