Which instrument is used to know the presence of an electric current in a circuit?

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.

$A$ magnetic compass shows a deflection when placed near a current-carrying wire. How will the deflection of the compass be affected if the current in the wire is increased? Support your answer with a reason.

If the key in the arrangement (Figure) is taken out (the circuit is made open) and magnetic field lines are drawn over the horizontal plane $ABCD$,the lines are

The direction of induced potential difference (induced current) is given by:

$A$ current $I$ passes through a circular loop $C$ and a solenoid $AB$ as shown below:
$(i)$ What is the polarity of: $(a)$ the face of the loop you are looking at? $(b)$ the end $B$ of the solenoid?
$(ii)$ What is the direction of the magnetic field at the centre of the loop?